The correct answer is C. went.
The simple past tense is used to talk about events that happened in the past. In this case, the event is “I went to Ooty on holiday.” This event happened in the past, so the simple past tense is used.
The other options are not correct because they do not express the past tense. Option A, am going, is in the present continuous tense. Option B, was going, is in the past continuous tense. Option D, had gone, is in the past perfect tense.
The present continuous tense is used to talk about events that are happening now. The past continuous tense is used to talk about events that were happening at a specific time in the past. The past perfect tense is used to talk about events that happened before another event in the past.
In this case, the event “I went to Ooty on holiday” happened in the past, so the simple past tense is used.