Select the grammatically correct phrase to complete the political journalist's article.

The public assumed the mayor resigned due to the recent embezzlement rumors. However, a sudden loss of support from her biggest donors _____ .

The correct answer is was what actually prompted her sudden exit.

This is an inverted wh-cleft sentence. In a standard wh-cleft, the wh- clause comes first (e.g., "What prompted her exit was a loss of support").

In the inverted version, the focused element ("a sudden loss of support") is moved to the front. It is then followed by the verb "to be" ("was") and finally the wh- clause ("what actually prompted her sudden exit"). Choosing "what actually prompted her sudden exit was" would incorrectly place the verb at the very end, leaving the sentence incomplete.