Which satirical element does this sentences use

  Understatement.

Explanation:

  Understatement is a satirical element use to intentionally make the situation seem less important than it really is.

  These types of figures of speech are satirical elements because an intense response is expected in front of the situation presented. In the example given in the question, the adjective pleased for receiving as a gift a new car full of 100 dollars bills is not the adjective that will be used in those situations.

  I hope this answer helps you.

i guess you are missing the idea that has to be supported, am i right?