First of all, in India there is no such law that abides you to follow any one religion of either parents.
In your case and specified scenario, the child can adopt the religion of either mother or father, there are no pressure requisite laws or norms set that may choose the religion.
The most common practice followed in Indian society is that the child gets the religion of his or her father as the surname of father is added behind the child's name.
But this is not at all a compulsory practice and following it or not is completely the discretion of the adopting parents.
Second of all, the best advice on selecting child's religion would be to select the religion that provides benefits to the child in future.
For example: If Mr. A is a christian (Minority) and Mrs. B is a Brahmin Hindu (Majority) then the child should preferably adopt the religion of father as in future he or she might get benefits of minority which will ease the struggle of success a little for the child.
Another example could be : If Mr A is a Hindu (general caste) and Mrs. B is a Muslim ( OBC) then the child should preferably adopt the religion of mother as in this case the OBC category would benefit the child in future as compared to generak category.