The honey bee is of greatest use to mankind as:

The correct option/answer is "Helps in cross-pollination".

To understand why this is the honey bee's greatest benefit to mankind, let's break down the role of honey bees step-by-step:
1. Primary Ecological and Agricultural Role (Pollination): Bees are the world's most important pollinators of wildflowers and crops. As they travel from flower to flower in search of nectar and pollen, they transfer pollen grains from the male anthers of one flower to the female stigma of another flower. This process is known as cross-pollination.
2. Impact on Food Security: A vast majority of flowering plants depend on animal pollinators to reproduce. Roughly one-third of the crops consumed by humans globally rely on pollination by insects, particularly honey bees. This includes fruits, vegetables, nuts, and oilseed crops. Without honey bees, agricultural yields would plummet, severely impacting the global food supply and ecosystem health.
3. Comparison with Other Benefits: While honey bees do provide direct commercial products such as honey, beeswax, and royal jelly (which have nutritional and medicinal benefits), these are secondary compared to the massive ecological and economic value of pollination. The survival of global vegetation and food crops far outweighs the direct value of honey production.

Therefore, helping in cross-pollination represents the honey bee's greatest and most critical contribution to mankind.