Step 1: Understanding the Question:
We must read the segmented sentence and identify which part contains a grammatical or idiomatic error.
Step 2: Detailed Explanation:
Let us analyze part (1): "We discussed about the problem so thoroughly".
The verb "discuss" means "to talk about" or "to converse regarding a specific topic".
Because the meaning of the word "about" is already inherently included within the definition of the verb "discuss", adding the preposition "about" immediately after it is grammatically redundant.
In English grammar, "discuss" is a transitive verb, which means it requires a direct object to receive its action directly.
Therefore, it should directly connect to the object "the problem" without any preposition acting as a bridge between them.
The correct usage is: "We discussed the problem so thoroughly."
Other common transitive verbs that follow this same rule (no preposition needed) include "describe", "order", "request", "emphasize", and "access".
For instance, we say "describe the scene", not "describe about the scene".
The other parts of the sentence (2, 3, and 4) are grammatically correct and connect logically to form a proper sentence structure.
Since the error lies in the redundant use of "about", part (1) is the correct answer.