Anna has identified a mutant that produces a protein that is much smaller than the wild-type protein. what type of mutation best explains this phenotype?

Frame-shift mutation occurs when the number of nucleotides inserted or deleted is not a multiple of three, thereby losing the actual location of the stop codon.  Frame-shift mutation could result in a longer or shorter protein.

Nonsense mutation occurs when a stop codon (the "nonsense" codon) is inserted at the wrong place inside a protein sequence, thereby producing an incomplete but shorter protein.  Most of the time, this results in non-functional proteins.

Reference: nature
Google and read-up on these mutations for more details.