Study the given pattern carefully and select the number that can replace the question mark (?) in it.

Logic: Add 1 to the product of the number of sides of the given figures to get the required number.

Figure 1: (5 $$\times$$​ 3) + 1 = 15 + 1 = 16

Figure 2: (6 $$\times$$​ 5) + 1 = 30 + 1 = 31

Figure 3: (3 $$\times$$​ 4) + 1 = 12 + 1 = 13

Similarly, follow the same pattern for the fourth figure:

So, the missing number is 21.

Thus, the correct answer is (a).