Explanation:

- The sentence "I was waiting from two hours" is incorrect. The preposition "from" is not properly conveying the intended meaning of waiting for a duration of time.

- Option 1: "since" is used to denote a starting point in time, but it doesn’t fit here because we are talking about a duration, not a starting time.

- Option 2: "in" is often used to refer to future periods (e.g., in two hours). It doesn’t work for this past continuous action.

- Option 3: "for" is the correct choice. It is used to indicate a period of time. The sentence should read: "I was waiting for two hours."

- Option 4: "at" is usually used for specific points in time, not durations, so it doesn’t fit here.