Someone who is very knowledgeable is said to know their what?

Here is the question :

SOMEONE WHO IS VERY KNOWLEDGEABLE IS SAID TO KNOW THEIR WHAT?

Here is the option for the question :

• Beans
• Oranges
• Onions
• Turnips

The Answer:

And, the answer for the the question is :

Explanation:

[STC009937]. If someone claims that you “know your onions,” it means that they believe you to have an in-depth understanding of a particular topic. In the 1920s, the phrase was initially used in print for the first time in American newspapers. Although there are some theories that link the slang to the famous British lexicographer C. T. Onions, it appears that the truth is that American teenagers in the 1920s merely enjoyed making up odd new words.