Correct choice is (c) sequence is not changed
The best I can explain: In the mutation process, the sequence is not changed (else it would no longer be an alignment), but gaps are inserted and rearranged in an attempt to create a better-scoring MSA. In the gap insertion process, the sequences in a given MSA are divided into two groups based on an estimated phylogenetic tree, and gaps of random length are inserted into random positions in the alignment.