The correct option is
A spot
The second sentence tells us that the subject 'he' is always very punctual. Hence, the first sentence should describe the action of such a person.
The phrase, 'spot on time' means exactly at the correct time. The sentence, 'He was spot on time' works well and describes the action of a punctual person. Hence, option A is correct.
The word, 'exact' is an adjective which modifies a noun. The blank has to be filled by an adverb, which modifies, the verb 'was'. The adverb form of 'exact' is 'exactly'. Hence the word 'exact' is not the appropriate answer and option B is incorrect.
The phrase, 'on time' means 'at the specified period of time'. While this phrase can be used on a standalone basis, the phrase, 'well on time' is not a common usage in English. Hence, option C is incorrect.
Option D is incorrect as option A is correct.