Final answer:
In the case of a trait with zero heritability, a pair of identical twins reared apart would have the least similar phenotypes since environmental factors are solely responsible for the variations in the trait. The correct choice is O a pair of identical twins, reared apart.
Step-by-step explanation:
If a multifactorial trait shows a broad sense heritability of 0, this means that variations in the trait among individuals in a population are due entirely to environmental factors and not genetic differences. In such a case, the set of people who will have the least similar phenotypes for this trait would be a pair of identical twins, reared apart.
Even though identical twins have identical genetic makeups, the entirely different environments in which they are reared can lead to very dissimilar phenotypes in a trait with zero heritability.
On the other hand, identical twins reared together, fraternal twins reared together, and siblings reared together would have more similar environments, which may result in more similar phenotypes.