The best choice to complete the text is (Choice C) "In fact,"
This transition clearly indicates that the statement following it is a supporting detail or fact that reinforces the importance of honeybee activities in relation to their impact on food crops and nutrition.
So the completed text would read:
"Honeybees play a crucial role in pollination, supporting both wild ecosystems and human agricultural systems. In fact, their activities affect 70% of the top human food crops, which supply about 90% of the world’s nutrition. Hence, their decline could have catastrophic consequences on global food security."