Genetic drift is a major cause of reduction in the gene pool. It refers to the random fluctuation in allele frequencies within a population. Genetic drift occurs when the size of a population is small or when a small group of individuals becomes isolated from the larger population.

Explanation:
Genetic drift can lead to a decrease in genetic diversity within a population and can result in the loss of certain alleles. This reduction in genetic diversity is known as a reduction in the gene pool. There are two main types of genetic drift:

1. Bottleneck effect: This occurs when a population undergoes a dramatic reduction in size, usually due to a natural disaster or human intervention. The surviving individuals may not represent the original genetic diversity of the population, leading to a reduced gene pool.

2. Founder effect: This occurs when a small group of individuals colonizes a new area and establishes a new population. The genetic diversity of the new population is determined by the genetic makeup of the founding individuals. If the founding individuals have a limited genetic diversity, the new population will also have a reduced gene pool.

Both the bottleneck effect and the founder effect can result in random changes in allele frequencies, leading to a loss of genetic diversity and a reduction in the gene pool. Over time, genetic drift can lead to the fixation of certain alleles and the loss of others, further reducing the gene pool.

In contrast to genetic drift, genetic mutation (option B) can actually increase genetic diversity by introducing new alleles into a population. Breeding programs (option C) and reproductive isolation (option D) may have an impact on the gene pool, but they are not the primary causes of reduction in the gene pool.

In conclusion, genetic drift is a major cause of reduction in the gene pool. It occurs due to random fluctuations in allele frequencies and can lead to a decrease in genetic diversity within a population.