The subordinate clause of the sentence uses the conjunction 'but'. This indicates a contrasting statement. Hence, we can draw the conclusion that the little sister was unable to draw, but is able now. Keeping this in mind "couldn't/was/can" in option C is the correct answer. It indicates the inability to draw in the past. Thus, 'couldn't/was' and the ability to draw now, thus 'can'.
"Could/can/can" in option A is incorrect as the first option 'could' rules out the complete option. 
"Can/was/could" in option B is incorrect as "can" indicates ability. However, 'but' indicates contrast. No one, due course of events, forgets to draw alone once they learn it (unless something drastic has happened, though no such conditions have been given).
"Was/can/could" in option D is incorrect as the auxiliary verb 'was' used here without a main verb is incorrect.
Thus, option C is the correct answer.