Question

A yellow leaf cannot prepare food why?

Answer 1

I think it is due to the absence of chlorophyll and hence absence
of photosynthesis

Answer 2

A yellow leaf cannot perform photosynthesis as it does not contain the photosynthesis performing pigment called chlorophyll.  

Chlorophyll is the only pigment present in a plant which is photosensitive and can trap and convert the “light energy” from the sun to “chemical energy” in the plant.

Yellow leaves on the other hand contain a pigment called xanthophyll which does not act as a catalyst to trap and convert “light energy” and thus, does not perform photosynthesis.