What happens when a mutation occurs during the formation of gametes in a parent sex cell?

most mutations will have no effect, although in rare cases an abnormal foetus may develop and even more rarely the embryo may be non-viable.

Explanation: A mutation at this stage is probably due to a random result of an external effect (cosmic ray for example) and probably a change in one nucleotide (a single nucleotide polymorphism). It is unlikely that the mutation will be an insertion, deletion or inversion.

Whatever the mutation, it is unlikely to occur in the relatively small proportion of coding DNA. If it does occur in coding DNA it is still unlikely to be at a critical codon. If it does occur at a critical codon, it may still code the the same amino acid, particularly if it is a third nucleotide.

There is a low probability that the mutation will prevent the formation of a fully functional protein, by altering a critical amino acid or altering the frame by changing a stop codon, or creating a stop codon, or altering a promoter region. Even in this case, any error can be avoided if the other allele is intact.

It is possible, however, that a mutation could result in a development abnormality (cystic fibrosis is an example),  or a non-viable embryo.

b is the best answer for this question.