He has been waiting __________ two hours.

The correct answer is for. This question tests the correct usage of prepositions of time, specifically distinguishing between "for" and "since." The preposition "for" is used to express a duration or a period of time. It answers the question "How long?" In the sentence "He has been waiting __________ two hours," "two hours" specifies the length of time that the waiting has been ongoing. Therefore, "for" is the appropriate choice. For example, you would say "for three days," "for a week," or "for many years." In contrast, "since" is used to indicate a specific starting point in time from which an action or state has continued. For instance, "since Monday," "since 2010," or "since he left." The other options, "from" and "at," are not used in this context to express duration. Mastering the distinction between "for" and "since" is fundamental for accurate temporal expressions in English.