Correct option is B. announced
Option B: 'Announced' means 'to make a formal public statement about a fact or occurrence.
This word forms a complete and meaningful sentence: The Election Commission made a formal public statement regarding the Assembly Election dates.
Hence this is the most appropriate word and option B is correct.
Option A: 'Pronounced' means 'very noticeable or marked'.
Option C: 'Promulgated' means 'promote or make widely known'.
Option D: 'Issued' means 'formally send out or make known'. It refers to sending out a written statement.
These words do not form complete meaningful sentences. Hence options A, C and D are wrong.