Because the challenge is asking for the smallest possible solution, this is a [puzzle]
I was unconvinced, so I had a look at the tag's summary and wiki entry:
A puzzle is a problem or enigma that tests the ingenuity of the solver. Solutions may require recognizing patterns and creating a particular order.
Puzzles are often contrived as a form of entertainment, but they can also stem from serious mathematical or logistical problems.
This doesn't seem to make it very clear which questions the tag should be applied to. On a broad reading, it could be applied to 90% of the questions on the site.
The obvious next step is to look at questions which have the tag, but that doesn't clear it up either. Some of them appear to have it because the question is a programming puzzle: e.g.
Others are about a puzzle: e.g.
What should puzzle mean? Do we need to split this tag? If so, what should we call the new tag(s)?