It's intuitive to me that the option "c) 2 ≤ i ≤ 12" is the only acceptable answer, since it is the only range you can tell immediately at a glance what numbers are included: 2, ..., 12. There shouldn't be a 1 or a 13 in the notation if there won't be a 1 or a 13 in the sequence it's describing -- it's just bad design.

If you select the option "c) 2 ≤ i ≤ 12", then Dijkstra's subsequent argument for zero-indexing becomes an argument for one-indexing, because a sequence of length N yields the range 1 <= i <= N when subscripting with 1, but 0 <= i <= N-1 when starting at 0. The latter is uglier than the former.