Concept of bioaugmentation

Bioaugmentation is defined as introducing specific, often pre‑selected or cultured, microorganisms into a contaminated environment to enhance pollutant degradation.
These added microbes may be bacteria, archaea, fungi, or consortia with known catabolic abilities for the target contaminant, and they can be used in situ or ex situ.

Option‑wise explanation

(1) Developing microbial strains through genetic engineering…

This describes strain development by genetic engineering, not the field application step of adding microbes to a contaminated site.
Genetically engineered strains could later be used in bioaugmentation, but the definition of bioaugmentation is the addition of such strains, not their laboratory construction. So this is not the best match.

(2) Ex‑situ bioremediation of toxins… by addition of selected microbes…

This describes one possible context (ex situ) and mentions adding microbes, but bioaugmentation is not restricted to ex‑situ systems; it includes in‑situ applications as well.
Because the definition in the question is general (no ex‑situ restriction), this option is too narrow and therefore not fully correct.

(3) Addition of nutrients at contaminated sites…

Adding nutrients, oxidants, or electron donors to stimulate native microbes is called biostimulation, not bioaugmentation.
Here, no new microbes are introduced; only the activity of indigenous populations is enhanced, so this option is incorrect for “bioaugmentation.”

(4) Addition of selected microbes both archaea and bacteria to the polluted site…

This exactly matches the standard definition of bioaugmentation:

• specific microbial cultures (bacteria, archaea, etc.) are introduced into the polluted site,

• with the purpose of enhancing the rate and extent of biodegradation.

Thus, option (4) correctly defines bioaugmentation.