reported speech......

The answer is:
[had not gone]

The answer must be " hadn't gone".

How to change '' didn't go '' in reported speech? -> 'hadn't gone'

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When we use reported speech, we are usually talking about the past (because obviously the person who spoke originally spoke in the past). The verbs therefore usually have to be in the past too. For example:

"I'm going to the cinema".
He said he was going to the cinema.

The tenses generally move backwards. In your case, past tense is changed to past perfect.