Krashen says the one "crucial and necessary ingredient" for acquisition is:

1. The input hypothesis puts comprehensible input at the centre. 2. Just any input is not enough; it must be understood by the learner. 3. Error correction, drills and rewards are not the necessary ingredient in this theory. 4. So the answer is comprehensible input. **Why not the others:** Error correction, grammar drill and rewards are teaching techniques; the theory names understood input as the necessary ingredient. _Source: Wikipedia, "Input hypothesis", section "Input hypothesis" (revision 1380165607)_