Final answer:
In the cycle of photopigment response to light, an enzyme converts cGMP to GMP after photoisomerization of retinal changes its shape, allowing visual transduction to occur in the retina.
Option is B).
Step-by-step explanation:
The cyclical process in which photopigments respond to light involves photoisomerization, where retinal changes shape from a cis to a trans form.
This process is catalyzed by an enzyme and is a key part of visual transduction in the retina. Specifically, when light strikes retinal, it changes from 11-cis-retinal to all-trans-retinal.
Subsequently, the all-trans-retinal is reduced and isomerized back to 11-cis-retinal, which can recombine with opsin to regenerate rhodopsin.
A relevant cycle step would be the conversion of cGMP to GMP by the enzyme phosphodiesterase, which is activated following the catalysis of phototransduction processes.
Therefore, the correct answer to the student's question is B) cGMP to GMP.
Option is B).