The correct decreasing order of water volume in these reservoirs is: Glaciers → Groundwater → Lakes and inland seas → Streams (R-Q-S-P).

Correct order of Earth’s water reservoirs

Most of Earth’s water is stored in a few major reservoirs, and their relative volumes are well studied in hydrology. When restricted to the four given categories (streams, groundwater, glaciers, lakes and inland seas), the decreasing order of water volume is:

• Glaciers (R)

• Groundwater (Q)

• Lakes and inland seas (S)

• Streams (P)

So the correct answer is:

(A) R-Q-S-P.

Why glaciers (R) come first

Glaciers and ice caps store the largest portion of Earth’s fresh water. A major share of the planet’s freshwater is locked as ice in Greenland, Antarctica and mountain glaciers, far exceeding all liquid freshwater bodies combined.

• Glaciers and ice caps contain the dominant fraction of freshwater after the oceans are excluded.

• Their total volume is thousands of times greater than that of rivers and streams.

Therefore, among the given options, R (Glaciers) must come first in decreasing order.

Why groundwater (Q) is second

Groundwater is the next largest freshwater reservoir after glaciers. It occupies pore spaces and fractures beneath Earth’s surface and represents a huge hidden storage compared with surface waters.

• Groundwater volume is much larger than the combined volume of all lakes and rivers.

• It forms the main source of usable freshwater for irrigation and drinking in many regions.

Thus Q (Groundwater) correctly comes after glaciers in decreasing order.

Why lakes and inland seas (S) are third

Lakes and inland seas hold a modest but still significant share of Earth’s surface freshwater. Famous examples include the Great Lakes, Lake Baikal, and the Caspian Sea (often classed as an inland sea).

• Total water in lakes is much smaller than in groundwater but larger than in flowing rivers and streams.

• Even the largest lakes by volume, such as Lake Baikal, are small compared with groundwater and glacial stores.

So S (Lakes and inland seas) correctly appears after groundwater but before streams.

Why streams (P) come last

Streams and rivers are the smallest of these reservoirs in terms of total water volume at any one time. They are dynamic “conveyor belts” that quickly move water rather than store large amounts.

• The total global volume of water in rivers and streams is tiny compared with lakes, groundwater, or ice.

• Their contribution to total freshwater storage is minimal, even though they are vital for ecosystems and human use.

Hence P (Streams) must appear last in the decreasing order.

Explanation of each option

Option (A) R-Q-S-P – Correct

• Order: Glaciers → Groundwater → Lakes and inland seas → Streams.

• This matches the known hierarchy of freshwater reservoirs by volume on Earth.

• Therefore, option (A) is correct.

Option (B) P-Q-R-S – Incorrect

• Order: Streams → Groundwater → Glaciers → Lakes and inland seas.

• This wrongly places streams first, even though they hold the least water of all four reservoirs.

• It also places glaciers below groundwater and streams, which contradicts the fact that glaciers are the largest freshwater store.

Option (C) S-P-R-Q – Incorrect

• Order: Lakes and inland seas → Streams → Glaciers → Groundwater.

• This puts lakes and inland seas above glaciers and groundwater, which is not supported by hydrological data.

• It also incorrectly places streams ahead of glaciers, even though streams hold far less water.

Option (D) R-P-Q-S – Incorrect

• Order: Glaciers → Streams → Groundwater → Lakes and inland seas.

• While it starts correctly with glaciers, it wrongly places streams before groundwater and lakes, despite streams having the smallest volume.

• Groundwater should come directly after glaciers and remain above lakes and streams.

Thus, considering the relative volumes of these freshwater reservoirs, R-Q-S-P (Option A) is the only scientifically consistent decreasing order.