Final answer:
The mutation that most significantly contributes to increasing gene pool diversity is gene duplication, as it can lead to new functions and alleles for natural selection.
Step-by-step explanation:
The type of mutation that plays the most important role in increasing the number of genes in the gene pool is D. Gene duplication. This mutation allows for the expansion of the genome over generations because when genes are duplicated, the extra copies can accumulate mutations without affecting the original gene's function.
This leads to the creation of new genes that can take on new functions and increase genetic diversity. Gene duplication is essential for evolution, as it provides a way for new alleles to arise and potentially confer selective advantages to organisms, which natural selection can then act upon.