A. Price equals total revenue.
B. Price equals average total cost.
C. Price equals marginal cost.
D. Price equals marginal revenue.

Helencpa- In case you didn't know, this was the week of NASBA Score Release.  Everyone has been obsessively F5'ing their scores over on their NASBA state sites to see their results.  That's why the other discussions have been quiet.  It is very hard to focus on anything else when scores are hanging over our heads.  I just finally got my BEC score less than two hours ago.  So now I can finally take a few minutes to offer a response to your question. 

I agree the answer is (c) because it applies to both short-term and long-term.  This is in Becker 2 Market Influences, subsection IX Market Structures and Pricing.  The condition for maximizing profit is MR = MC, as noted by the poster above me.  To maximize short-run profits, competitive firms must produce at the output rate where P = MR = MC.  The long-run equilibrium position for a competitive firm is where P = MC = Minimum ATC; firms are producing where price equals minimum average total cost.  So we see that the equation applicable to both scenarios is P= MC.  The answer can't be (a), (b) or (d) because of the "always" in the call of the question.