Answer : 3. "stoke"
Explanation :

“Stoke” means “add coal or other solid fuel to (a fire, furnace, boiler, etc.)”. The word “stoke” fits the blank perfectly.

“Soak” and “strike” do not make any sense here either grammatically or contextually.

“Soak” means make or allow (something) to become thoroughly wet by immersing it in liquid

“Strike” means hit forcibly and deliberately with one’s hand or a weapon or other implement

“Increase” may also fit the blank, but “stoke” is more appropriate in the given context.