17) D

1 – We know that statement 2 could definitely be a reason. If the sperm carried the allele and one allele is enough to cause the condition, then 2 could be reason. We can eliminate B and E. 

2 – If the condition is dominant, then P, Q, R and S must be recessive. Since T has condition, it must have at least one dominant allele. Therefore, the reason why U has the condition is because, it obtained the allele from T. 

3 – If the egg from S had a mutation, then it certainly could have caused the condition. 

All statements could be reasons why U has the condition. 

Hence, D is correct.