Reinsurance is a strategy used by insurance companies to manage longevity and mortality risks. It involves transferring a portion of the insurer's risk portfolio to another party, known as the reinsurer. The reinsurer agrees to cover the losses exceeding a certain amount, thereby reducing the likelihood of the insurer having to make large payouts when the mortality experience turns out to be worse than expected. This strategy is particularly effective in managing longevity and mortality risks as it allows the insurer to spread the risks and protect itself from potentially catastrophic losses. Furthermore, reinsurance can provide the insurer with greater capacity to underwrite more policies, thereby increasing its profitability. It also enables the insurer to stabilize its financial results by smoothing out the peaks and valleys of underwriting cycles.

Choice A is incorrect. Adding a substantial risk premium to the final break-even premium payable may not be an effective strategy for managing longevity and mortality risks. This is because it could potentially make the insurance product unaffordable for many customers, leading to a decrease in demand and hence, profitability.

Choice B is incorrect. Avoiding high-risk business might reduce exposure to longevity and mortality risks but it also limits the potential for higher returns that come with higher risk. Moreover, this strategy does not manage the risk but rather avoids it altogether which is not always feasible or desirable in the insurance industry.

Choice D is incorrect. Using a high interest rate can increase investment income but it does not directly address longevity and mortality risks which are related more to life expectancy trends than interest rates. Furthermore, using a high interest rate could lead to other financial risks such as inflation risk or interest rate risk.