"Get by" means to manage sufficiently. This is your answer. "I know enough English to get by in an everyday situation." or "I know enough English to get by in everyday situations." In this example, if you use "get across" (as in "convey"), you need to specify WHAT you are getting across, eg. your message. "I know enough English to get my message across in an everyday situation."

Get by. Example: Get (there) by train Get across. Example: Get across to another building. (It's like getting to another side of somewhere. Be it a person or a thing.)

both seem to be correct, but one of them must be selected as the correct answer.

get by : passed a situation

get across: clarify come across:find by accident