Concept:The adjective "jealous" is correctly followed by the preposition "of" when pointing to the person or thing someone envies.
Explanation:The sentence to complete is: "Why are you always jealous ____ other people?"
In standard English, the correct collocation is "jealous of".
This means feeling envious or resentful toward someone because of their qualities or possessions.
For example: "He is jealous of his brother's success."
The options "at", "about", and "with" do not collocate correctly with "jealous".
Therefore, "of" is the only appropriate choice here.
Answer:A. of — The completed sentence is: "Why are you always jealous of other people?"