**Explanation:**

When we say "I suddenly realized that I ---- (meet) her at school," the verb tense to be used depends on the context and the sequence of events. In this case, the correct answer is "had met."

**Past Perfect Tense:**

The past perfect tense is used to talk about an action that happened before another action in the past. It is formed by using "had" followed by the past participle of the verb.

**Sequence of Events:**

In this sentence, the speaker is reflecting on a past event (meeting someone at school) after realizing it. Let's break down the sequence of events:

1. The speaker met someone at school in the past. This is the first action that happened.
2. The speaker suddenly realized this fact. This is the second action that happened.

Since the realization happened after the meeting, we need to use the past perfect tense to show the chronological order of the events.

**Using "Had Met":**

By using the past perfect tense, we can say:

"I suddenly realized that I **had met** her at school."

Here, "had met" emphasizes that the meeting took place before the realization. The past perfect tense helps to establish a clear timeline of events.

Overall, the use of "had met" in this sentence indicates that the meeting happened before the realization, which is the correct tense to convey the intended meaning.