Explanation:

The correct answer to the given sentence is "had not been working." This answer is in the past perfect continuous tense, which is used to express an ongoing action that started in the past and continued up until another point in the past.

Breakdown of the sentence:

The sentence can be broken down into two parts:

1. The action of not working all day: "I ____________ (not / work) all day."
2. The consequence of not being tired and going to the disco: "So I wasn't tired and went to the disco at night."

First part:

The first part of the sentence talks about the action of not working all day. To express this in the past perfect continuous tense, we use the phrase "had not been working." Let's analyze this phrase:

- "I" is the subject of the sentence, referring to the speaker.
- "Not / work" is the action being described. The negative form "not" is used to indicate the absence of the action.
- "Had not been" is the past perfect continuous tense of the verb "to work." It indicates an ongoing action that started in the past and continued up until another point in the past.

Second part:

The second part of the sentence explains the consequence of not working all day. It states that the speaker wasn't tired and went to the disco at night. This part simply provides the reason for the speaker's actions based on the fact that they hadn't been working all day.

Overall meaning:

The complete sentence suggests that because the speaker had not been working all day, they weren't tired and decided to go to the disco at night.

In conclusion:

The correct answer "had not been working" accurately conveys the past perfect continuous tense and effectively communicates the relationship between the speaker not working all day and their subsequent actions.