Correct answer: (4) Clostridium

Methanol as an industrial fuel can be produced biologically via certain anaerobic bacteria, especially species of Clostridium, which are known for solvent-producing fermentations (including alcohols) and are classically linked with exam questions on microbial fuel alcohol production.

Option-wise explanation:

• (1) Acetobacter

  • Acetobacter are aerobic, acid-producing bacteria that oxidize ethanol to acetic acid (vinegar).

  • They are not used for industrial methanol fuel production; they tend to oxidize alcohols further rather than generate methanol.

• (2) Azotobacter

  • Azotobacter are free-living nitrogen-fixing bacteria found in soil.

  • Their significance lies in nitrogen fixation and biofertilizer use, not in alcohol or methanol fuel production.

• (3) Saccharomyces

  • Saccharomyces (budding yeast) is the classical organism for producing ethanol, not methanol, through sugar fermentation.

  • Yeasts can generate trace methanol from pectin in fruit mashes, but they are not used for bulk methanol fuel production.

• (4) Clostridium

  • Clostridium species are anaerobic, solvent-producing bacteria (e.g., ABE fermentation for acetone–butanol, and other alcohols).

  • In standard microbiology/biotech MCQs, Clostridium is the organism associated with microbial production of industrial fuel alcohols, including methanol in this context, making option (4) the expected correct answer.