The arrow represents a set of calculations that are common across all the given equations. Identify the calculations involved and solve the fourth equation on the same basis:

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Logic: (1st number $$\times$$ 2nd number) + 1 = Result

I. 4 $$\times$$ 5 = 20, 20 + 1 = 21

II. 7 $$\times$$​ 8 = 56, 56 + 1 = 57

III. 6 $$\times$$​ 9 = 54, 54 + 1 = 55

Similarly,

IV. 5 $$\times$$​ 3 = 15, 15 + 1 = 16

Thus, correct option is (d).