The researcher isolated a different single cell and determined that it was producing a certain protein. However, the protein was much shorter than it should have been. What kind of mutation likely occurred in this protein?

Nonsense mutation.

Explanation:

A nonsense mutation, also called stop mutation, is a change in DNA that produces an earlier stop codon (UAG, the amber codon, UGA, the opal codon, or UAA, the ochre codon on RNA, and TAG ("amber"), TAA ("ochre") or TGA ("opal" or "umber") in DNA.

It means, it causes a protein to terminate its translation earlier than expected. This causes a shortened or nonfunctional protein to be expressed.