Final answer:
The term for the size of an ideal population that loses genetic variation at the same rate due to genetic drift as the actual population is called the Effective population size.
Step-by-step explanation:
The term that describes the size of an ideal population that loses genetic variation due to genetic drift at the same rate as the population under study is known as the Effective population size (A). Neither the Genetic bottleneck effect (B), Allelic fixation (C), nor the Founder effect (D) is correct in this context, as they all represent different phenomena related to genetic drift, but not the concept described by the question.
Genetic drift is more significant in small populations and can be magnified by events like natural disasters, which is known as the Bottleneck effect. This effect results in a large portion of the genome being suddenly wiped out, which can drastically change the genetic structure of the surviving population. Similarly, the Founder effect occurs when a small group from a larger population starts a new one, potentially carrying a non-representative sample of the larger population's alleles, leading to a change in the genetic structure.