(a) Both, Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
In reptiles, temperature determines sex of offspring because sex is not genetically determined in such animals — R correctly explains A.
Both statements are factually correct as per the textbook: in some reptiles, incubation temperature determines sex, and in animals like snails, sex is not genetically determined. R directly explains why A is true, making option (a) the correct choice. Students must check whether R correctly explains A, not just whether both are true.