Stock markets ______at the news of the coup.

The correct answer is plunged.

Explanation:

To choose the correct word to complete the sentence, let's analyze the context and the meanings of the given options:

1. Plunged: Means to drop sharply, decrease rapidly, or fall suddenly. In financial contexts, stock markets "plunge" when there is sudden political instability, such as a coup, leading to rapid market decline and investor panic.

2. Plugged: Means to block or fill a hole, or to promote something.

3. Poised: Means having a composed and self-assured manner, or ready to take action.

4. Probed: Means physically explored or thoroughly investigated.

Therefore, plunged is the most appropriate word to describe the sudden decline in stock markets caused by bad news like a coup.