Missense mutations involve a nucleotide substitution resulting in changed amino acids. Sometimes the effects of missense mutations may be only apparent under certain environmental conditions; such missense mutations are called conditional mutations. Many missense mutations result in proteins that are still functional, at least to some degree.

Also, all the other answers would probably leave you with either a greatly altered or non-functional protein.

Missense - Nucleotide substitution results in CHANGED aminoacid..like what happens on Sickle cell disease, substitution of glutamic acid with valine..

the question says requires exogenous tryptophan for growth at 42 but no at 30.. so its still partially functional,so it cant be NONSENSE.