The rectangle 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:

7 ▬ 4 = 27 / 4

6 ▬ 1 = 5

4 ▬ 3 = 11 / 3

a ▬ b = ?

Check for first: (7 ▬ 4)
According to option (d),
→ (ab – 1) / b
= (7 × 4 – 1) / 4
= (28 – 1) / 4
= 27 / 4  (Matches!)

Check for second: (6 ▬ 1)
→ (6 × 1 – 1) / 1
= (6 – 1) / 1
= 5  (Matches!)

Check for third: (4 ▬ 3)
→ (4 × 3 – 1) / 3
= (12 – 1) / 3
= 11 / 3  (Matches!)

Thus, the pattern is perfectly matching with Option (d).