The answer is: D. understatement.
An understatement is the display of something as minor, worse or less significant than it actually is. In this case, the idea is treated casually or downplayed, which means somebody makes it appear less important than it is.
The rest of the options are incorrect because a hyperbole is an exaggeration not meant to be taken literally; a paradox is a statement that sounds reasonable but its conclusion does not make sense or is contradictory, and an irony means the opposite of what is being said.
