Topic: Sequence of Tense,

Best Answer Explained in Below-:

The sentence "I opened the door as soon as ___ I the bell" requires a verb form that indicates a completed action in the past. The phrase "as soon as" typically introduces a clause that describes an event that precedes or coincides with another action. In this case, the action of opening the door is contingent on the moment the bell was heard.

Option D, "heard," is the past simple form of the verb "to hear." It effectively indicates that the action of hearing the bell occurred in the past and thus provides a clear temporal relationship with the subsequent action of opening the door.

In contrast, the other options do not fit grammatically or contextually. "Have heard" (A) implies a present perfect tense, suggesting an action related to the present, which doesn't align with the past-focused event of opening the door. "Was hearing" (B) uses the past continuous tense, which indicates an ongoing action in the past; however, this does not capture the moment of realization that triggered the door opening. Lastly, "am heard" (C) is in the present passive voice and is not appropriate for past actions.

Thus, "heard" is the correct choice, as it succinctly conveys that the bell was heard prior to opening the door.