Select the most appropriate option to fill in the blank.
This class used to be very noisy, but it was unusually ______ today.

The conjunction 'but' introduces a contrast to the fact that the class used to be "very noisy". Therefore, we need a word that implies quietness or calmness.
Subdued: (Of a person or their manner) quiet rather than reflective or depressed.

Excited, Rowdy, Screaming: All relate to high energy or noise, which do not create the necessary contrast.

Thus, 'subdued' is the correct choice.