Mortality risk is the most significant risk faced by life insurance companies. This risk arises when policyholders die earlier than expected, which could be due to various reasons such as illness, disease, or fatal accidents. When this happens, the insurance company is forced to make life insurance payouts earlier than anticipated. This can lead to significant financial losses for the company, especially if the number of early deaths is higher than what was initially projected when pricing the insurance policies. Therefore, managing mortality risk is a critical aspect of the operations of a life insurance company.

Choice A is incorrect. While low rates of interest can impact the profitability of a life insurance company, they do not pose the most significant threat to a whole life insurance business. This is because these companies have strategies in place to manage interest rate risk, such as asset-liability management.

Choice C is incorrect. Longevity risk, which refers to the risk that policyholders live longer than expected, can indeed affect profitability. However, it does not pose the most significant threat for whole life insurance businesses as these policies are designed with this risk in mind and premiums are set accordingly.

Choice D is incorrect. Currency risk could potentially impact an international life insurance company's profitability if it has operations in different countries with varying currencies. However, this risk isn't typically considered as significant for a whole life insurance business compared to mortality risk.