which is a general representation of a secondary amine?

Brief Explanations:

A secondary amine has a nitrogen atom bonded to two organic - groups ($R^1$ and $R^2$) and one hydrogen atom. The first structure is ammonia, the second is a primary amine (one organic - group and two hydrogens on nitrogen), and the correct structure for a secondary amine is the one with two different organic substituents ($R^1$ and $R^2$) on the nitrogen along with one hydrogen.

Answer:

The structure with nitrogen bonded to $R^1$, $R^2$, and $H$ (the third structure in the list).