The correct answer is B. Sedate.
A. Depressive is not a synonym of staid. Depressive means feeling or showing sadness or low spirits. Staid means serious and respectable in manner or appearance.
B. Sedate means calm and relaxed. This is a synonym of staid, as both words describe someone who is not excited or lively.
C. Backward is not a synonym of staid. Backward means slow to develop or progress. Staid means serious and respectable in manner or appearance.
D. Dull is not a synonym of staid. Dull means not interesting or exciting. Staid means serious and respectable in manner or appearance.