The best way to explain this:

Firstly, the only possible combinations for the child are 1,2 or 2,3, one from each parent (that eliminates A, C & D)

They are looking for the genotype for III if he is unaffected by the disease. In I, we see that the male is affected, and the female is not. In II, both the female and male are affected.

If II female is 1,1 one of each 1 alleles comes from each parent, II male, we recognize that the 3 allele has to come from I female, and 1 allele HAS to come from I male. Therefore, one can infer that it is the one of the 1 genes from the I male causing the disease. If III is unaffected, the only option would be 3,2.

The unaffected person will have the alleles from their parents not present in the diseased people. This is alleles 2 and 3.

think this pedigree is for X-linked recessive disease. since the disease-causing allele is 1. and in the case of X-linked recessive diseases, only one allele is enough for causing disease in MALE.(evident by II male). And in the case of FEMALES , two alleles are needed. evident in this pedigree ( II female)

There's 3 alleles present - 1, 2, and 3. The question asks what the child of a 1,3 x 2,2 pairing would be. Thus, it'd have to be either 1,2, or 2,3 because they have to inherit one of the #2 alleles from momma. From there, we see that all males with the 1,3 genotype are affected - don't get distracted by the 1,3 female! Since the question is asking for an unaffected male, the answer can't be 1,2, making his genotype 2,3. But silly question imo ¯_(ツ)_/¯

I think the unaffected 1,3 (I, female) is due to incomplete penetrance