“For q12 in S3 can option D also be correct? As the 'linker' is linking carbons doesn’t that make it a linker carbon anyway?”
Good question!
Yes, the 2 carbons from (c) are definitely linker carbons. But, answer choice D says that they must BOTH be secondary positions. Certainly, one of the linker carbons will be secondary, but the other links to nitrogen, so you would have R-CH2-N, which is a primary carbon.