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.