The correct option is
D had gone
A sentence is in the type 3 conditional refers to an unreal past condition (if-clause) and its probable result (main clause).
The
given sentence shows a type 3 conditional, as the if-clause tells an
unreal past action and the would-clause tells its probable result.
In such a sentence, the tenses are:
if clause - if + past perfect
and
main clause - perfect conditional/perfect continuous conditional
The main clause "his vacation would have been better" is in the perfect conditional, hence the if-clause is in the past perfect tense.
Option D: 'Had gone' is in the past perfect tense, hence it's the most
suitable option for the given sentence, and option D is correct.
Option A: 'Went' is a simple past tense verb whereas the if-clause will be in the past perfect tense.
Options B and C: 'Have gone' and 'has gone' are in the present perfect tense, whereas the main clause is in the past. The present perfect tense states an action that has just completed, but here the action is a probable result in relation to a past conditional.
Hence options A, B and C are incorrect.