Explanation:

- Fillip is a word with several meanings, primarily as a noun, it means a stimulus or boost to activity. It can also refer to a flick of the finger.

- Option 1: Boost

- This is a correct synonym for "fillip" when referring to a stimulus or incentive.

- When you give someone a boost, you're providing encouragement, or helping them by supporting or lifting them in some way.

- Correct Answer

- Option 2: Blow

- Generally means an impact or hit, not a correct synonym for "fillip."

- Option 3: Flip

- Refers to turning something over quickly or a light switch, not related to the meaning of fillip.

- Option 4: Knock

- Usually involves a strike or hit, not a correct synonym for "fillip."