A triad that contains the notes A and C is an A minor triad (A, C, and E), as A and C are the root and minor third of the triad, respectively.
To complete the triad, the note that could logically fit is E, which would form the A minor triad (A, C, E).
Thus, from the options given, the correct answer is E. The option "F or E" is inaccurate because F would not fit into the A minor triad context.