Normal sexual behavior is that which

a. reflects the average or median behavior of a group

b. is supported by religious teachings

c. reflects the preferences of those of high status in a society

d. leads to successful reproduction

a. Represents the average or median behaviour of a group.

Explanation:

Any 'normal' behaviour denotes the usual aspects of the variable / attribute, as represented by average for a group of people.

Examples : Normal Heart Beat - 70 to 80 is a range decided around average after examining various people's heart beat.

Similarly, Normal Sexual Behaviour is average behaviour attributed after evaluating sexual behaviour of various people.