When we look carefully at the sequences provided que have that there is a substitution in nucleotide 7, 5'-->3', where it should be a Cytocine is a Thymine, this type of mutation is called substitution. Now for this mutation to be part of an individual without any parent being a carrier, it means that the mutation occurred prior to fertilization or in the first stages of the zygote. Taking this into consideration we can say that the correct answer is option B.