Answer:
The correct answer is to please. It is used as a complement in the way of infinitive clause.
Step-by-step explanation:
This type of structure where “it is + adjective or noun to” can be seen is used to express an opinion or advice.
In the case of the sentence That type of person is hard to please, we are expressing an opinion about a type of person.
That type of person: it
Is
Hard to please: adjective to
As we can see it complies with said structure, which serves to highlight the information.
If we wanted to use it with a noun instead of an adjective we could say for example:
It is a pleasure to please that type of person.