Final answer:
An allele increases in frequency within a population if it leads to higher reproductive success or behaviors that promote reproductive success of those carrying it. The increase through longevity but with lower reproductive success is less likely to contribute to allele frequency increase.
Step-by-step explanation:
An allele can increase in frequency in a population if, on average, individuals in the population that have the allele have higher reproductive success than individuals with other alleles. This process is known as natural selection. Another scenario is when the allele causes behaviors that result in other individuals in the population that carry the allele having higher reproductive success. In this case, the beneficial allele gets propagated through the population.
However, if an allele increases longevity but results in lower average lifetime reproductive success compared to other alleles, it may not necessarily increase in frequency over time. Reproductive success is crucial because it contributes to the next generation's gene pool. Therefore, the correct answers are scenarios 'a' and 'b' which discuss higher reproductive success and behaviors increasing reproductive success, respectively.