The correct answer is (D) To make active. In chemistry, to make a catalyst, carbon, or other molecule active, or facilitate or hasten a chemical reaction. To aerate sewage in order to hasten decomposition. In physics, to excite or make radioactive. Also, to place a military unit on an active status or resurrect to a state of readiness any dormant organization.

Examples of use:

• A beam of neutrons activated the reaction.
• His regiment was activated for imminent deployment.