Correct Answer: a drop in the ocean

Solution : The first option is correct.

The phrase a drop in the ocean is an idiom that conveys the idea of something very small or insignificant compared to a much larger thing. In this case, it emphasises that the speaker's knowledge about archaeology is relatively small compared to their sister's knowledge.

The meanings of other words are as follows:

• A spit in the ocean : This phrase is similar to "a drop in the ocean" and means something very small or insignificant compared to a much larger context.
• A fish out of water: This expression refers to someone who feels uncomfortable or out of place in a particular situation or environment.
• Water under the bridge : This phrase refers to past events or issues that are no longer relevant or important because they have been resolved or forgotten.

Therefore, the complete sentence would be: My knowledge about archaeology is a drop in the ocean in comparison to my sister's.