The correct answer is A. for.
Pine means to long for or desire something very much. In this sentence, we are saying that we often do not get what we long for. The preposition “for” is used to indicate the object of a person’s desire. For example, “I pine for you” means that I long for you.
The other options are not correct because they do not make sense in the context of the sentence. “To” is used to indicate the purpose of an action. For example, “I went to the store to buy some milk.” “On” is used to indicate a location or position. For example, “The book is on the table.” “About” is used to indicate a topic or subject. For example, “I am talking about the weather.”
In conclusion, the correct answer is A. for.