Solution

Let us try to understand the meaning of the sentence from the partial information.

The sentence states that the parliament made a certain kind of return on Monday, and the senators were chirpy and guffawed through the question period. The words chirpy (cheery) and guffaw (chuckle/laugh) tell us that the mood in the parliament was lively and light, so the fill-in, which describes the kind of return that the parliament made, must be a positive word that means the environment was "happy" and "exuberant". The word exuberant can be a good fill-in.
Now, let us look at the meaning of the adjectives in the options.

A. lugubrious – looking or sounding sad and dismal
B. rambunctious – uncontrollably exuberant; boisterous
C. opprobrious – expressing scorn or criticism
D. staid – sedate, respectable, and unadventurous
E. unflagging – tireless; persistent
F. ebullient – cheerful and full of energy
Both 'rambunctious' and 'ebullient' fit in the blank well. The words 'lugubrious' and 'opprobrious' are negative and therefore do not fit in the blank. 'Staid' and 'unflagging' are unrelated to the meaning of the sentence and therefore do not convey the correct meaning.

The correct answers are B and F.