Let the dominant allele be: T
Let the recessive allele be : t
If only U has a recessive condition, their genotype must be tt. This means that S and T must be heterozygous.
Since we want the minimum number of heterozygous people, we must assume that R has to be TT.
In order for S to be Tt and R to be TT, only one of P and Q needs to heterozygous.
This means that if only U has the recessive people, 3 people must be heterozygous.
If R and U both have the recessive condition, P and Q must both be heterozygous.
This means that if R and U have the recessive condition, 4 people (P, Q, S and T) must be heterozygous. Hence, E is the answer.