Insertion of an additional base at start codon make the protein still functional?
I think the question is fairly confusingly worded, so good job on your attempt based on that.


If a C is inserted within the start codon, I presume that is referring to making a sequence that was originally ATGXXX and making it either ACTGXXX or ATCGXXX. Either insertion means the start codon will be missing completely. This is going to either cause no protein to be made whatsoever, or for the beginning of the protein to be severely truncated or completely frameshifted if there is another open reading frame created downstream. This is going to be magnitudes more serious than a deletion of 3 bases corresponding to a lysine.
I think your answer here is fine - lots can happen with a missing stop codon. If there happens to be another stop codon in the same reading frame shortly downstream, then the effect could be not that severe, but if not it could result in a substantial additional peptide chain added to the end of the protein, which could cause all sorts of issues with folding, and the mRNA may not even be functional and be degraded.


Again, this question isn't worded in the best way, but I think the general principle it is trying to convey is that whole missing codons do a lot less damage to the structure of a protein than mutations that affect the reading frame or the stop/start codons, because they affect only a single amino acid. Of course, it could turn out that that amino acid is crucial to the structure of the protein, but the question is about what is most "likely."