Correct option is (a) 420 x 297
Easiest explanation: The size of the designation A3 in mm is 420 x 297. The designations A0, A1, A2, A4 and A5 have sizes 1189 x 841 mm, 841 x 594 mm, 594 x 420 mm, 297 x 210 mm and 210 x 148 mm respectively. Standardizing helps in uniformity of the products all over the nation and will avoid local variations.