This requires the knowledge of genotypes and the X and Y chromosomes.
Let the gene that causes the condition be Xª and the good gene be Xⁿ
We need to starts with the male with NPS in the first generation. Since he has the condition, he must have the bad gene Xª.
He passes this gene to his daughters, 8 and 9. Since his daughters do not have NPS, the man’s wife (6) must be have provided the good gene. Therefore, 8 or 9 have to be heterozygous.
The only pairs given with 8 and 9 are D and E. We now need to find out who out of 4 and 5 are heterozygous.
We know 3 does not have the condition. So he must have Xⁿ. Since one of his sons has the condition, the mother (4) must have provided the faulty gene. She could not be homozygous with the faulty gene as this would cause her to have the condition. She could not be homozygous with the good gene as she would not have been able to get an offspring with a condition (but she does have a offspring with NPS). So 4 must be heterozygous.
D is the answer.