‘Pull your socks up’ means:
Correct answer is: D. To improve
The idiom 'pull your socks up' means to make an effort to improve performance.
A piece of cake: This idiom means something is very easy to do. It's like a piece of cake is easy to eat, so completing the task is effortless.
You can't have your cake and eat it too: This idiom means you can't have both of two desirable things at the same time. It's like if you eat your cake, you no longer have it.