The limiting factor which affects both populations about equally is competition within the population for living space and is therefore denoted as option D.
This is defined as a type of rivalry between organisms or species for resources such as food, space etc in a particular area over a given period of time.
Since we were told that the the bison population is significantly below carrying capacity and the pheasant population has exceeded the carrying capacity in the same nature reserve then the competition for space affects both equally which is why increase in both has been effectively managed.
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