The core grammar rule tested here is the superfluous (unnecessary) use of prepositions.
1 The Rule of the Verb 'Discuss':
The word "discuss" is a transitive verb, which means it directly requires an object to follow it without the help of a preposition. The definition of discuss itself means "to talk about." Therefore, adding the preposition "about" right after "discuss" makes it redundant (e.g., saying "discussing about" is grammatically equivalent to saying "talking about about").
2 Correct Structural Uses:
Active Verb Form (No Preposition): "We discussed the matter." (Correct)
Noun Form (Requires Preposition): "We had a discussion about/on the matter." (Correct)
