How do you cherry-pick a merge commit?
Add -m 1 (or -m 2) to say which parent is the mainline, because a merge commit has two parents and Git cannot know which side's diff to copy.
Explain it simply
A merge commit is a junction with two incoming roads. 'Copy what this commit changed' is ambiguous: compared with which road?
With -m 1, Git diffs against the first parent, which is the branch you merged into. That produces the feature's net changes. Using -m 2 diffs against the branch that was merged in, which is rarely what you want. The same flag is needed for reverting merges. Most teams avoid this by cherry-picking the feature's individual commits or using squash merges.
Example
git show --format=%P -s 9c1d2ef # two parent hashes
git cherry-pick -m 1 9c1d2efWhat to say in the interview
"Merge commits have two parents, so cherry-pick needs -m to choose the mainline, usually -m 1. Without it Git errors out. I would usually avoid it and pick the individual commits instead."