C is the only right answer offered. A, alumnum, is a word I made up; doesn't exist, but sounds enough like singular that it might've caught you if you weren't sure. B, alumni, is the most frequently misused version; it's the plural form, always, only, and therefore you can't say "I'm an alumni of X University." C, an alumnus, refers to either an individual male graduate OR any individual graduate. The female equivalent is alumna (not offered as a choice as it's also individual), but women can also use alumnus to describe themselves. Similarly, D, alumnae, is the plural of alumna. More than one female graduate.