Select the most appropriate option to fill in the blank.

Scientists all over the world are engaged in ______ how to find a vaccine for COVID-19.

researching

The most appropriate option to fill in the blank is:

Option 1: researching

Explanation: The sentence is discussing the activities of scientists related to finding a vaccine for COVID-19. The word "researching" accurately describes the process of conducting systematic study and investigation to gather information and make progress towards a solution.