Stock markets ______at the news of the coup.

The correct option is plunged.

Let us break down the meaning of the words to understand why "plunged" is the most appropriate fit for the sentence:
1. Plunge: To fall or drop suddenly and quickly. In financial and economic contexts, stock markets are said to "plunge" when stock prices drop rapidly and significantly due to bad news, instability, or panic.
2. Plug: To block or fill a hole, or to advertise something. This does not make sense in the context of stock markets reacting to news.
3. Poise: To be balanced, suspended, or prepared for action. While markets can be poised for a change, "poised at the news" is grammatically and contextually incorrect here.
4. Probe: To physically explore or examine something closely, or to investigate. Stock markets do not probe at news.

A coup (a sudden, violent, and illegal seizure of power from a government) typically creates massive political and economic instability. Such negative news causes investors to panic and sell off shares, leading to a sudden, sharp decline in stock values. Therefore, saying the stock markets plunged at the news of the coup is the most accurate and logical choice.