Which of the following is the main source of food for stream-inhabiting organisms?

Correct option is B

Correct Answer: (b) Allochthonous substance
Explanation:
→ Streams (especially shaded, headwater streams) receive most of their energetic input as allochthonous material — leaf litter, terrestrial detritus and particulate organic matter from the watershed.
→ Shredders and collectors process this material, forming the base of the stream food web.
Information Booster:
→ Autochthonous production (algae) gains importance in open, well-lit reaches but not in heavily shaded headwaters.
Additional Information (Other Options):
(c) Not a standard term; (d) Heleoplankton: still-water plankton, irrelevant in flowing streams.