The committee's decision, which surprised many observers was met with mixed reactions from the public.
Correct: B
The clause 'which surprised many observers' is a non-essential relative clause. Non-essential clauses provide additional information that is not critical to the meaning of the sentence and must be set off by commas. Since there is already a comma before 'which,' a second comma is needed after 'observers' to complete the parenthetical phrase. Option B correctly adds this comma.