The phrase you provided refers to understanding the connection or relationship between two ideas: students' amount of homework and their test scores. This pertains to Clarity and Coherence, as you are expressing a need for clear understanding and logical connection between concepts.
On the other hand, Syntax and Diction would involve the structure of the sentence and the choice of words, which isn't the primary focus of your concern in this context.
So, your inquiry is about clarity and coherence.