Due to _________, the subways were closed all morning.floodare floodedfloodsits flooding

The correct answer to fill in the blank is "flooding."

In this context, "flooding" is a gerund that refers to the ongoing action of water covering an area, which is an appropriate cause for the closure of the subways. The sentence would read: "Due to flooding, the subways were closed all morning."

To briefly explain the reasoning:

1. Grammatical Structure: The word that fills in the blank must function as a noun because it's introduced by "due to."
2. Meaning and Usage: "Flooding" as a noun encapsulates the situation that led to the subway closure, indicating that it was the action of water overflowing that caused the disruption.

Therefore, "flooding" correctly fits and conveys that the subways were closed for that specific reason.