The correct answer is C. am going.
The sentence is in the present progressive tense, which is used to talk about actions that are happening now or will happen in the near future. The other options are not in the present progressive tense.
A. went is in the simple past tense, which is used to talk about actions that happened in the past.
B. have gone is in the present perfect tense, which is used to talk about actions that have happened or been completed in the past.
D. going is in the infinitive, which is the basic form of a verb and is used to talk about actions in general.
In this case, the most appropriate word to use is “am going” because it is the only option that is in the present progressive tense. This tense is used to talk about actions that are happening now or will happen in the near future. The sentence “I am going to the dentist tomorrow” means that the speaker is planning to go to the dentist on the following day.