the cost of the energy consumed, if the rate is ₹ 10 per unit.

Given: energy consumed = 5.5 kWh (from part (b)), rate = ₹10 per unit

One unit = 1 kWh, so 5.5 kWh = 5.5 units.

Cost = number of units × rate per unit = 5.5 × 10 = ₹55

Cost of the energy consumed = ₹55

A "unit" on an electricity bill is exactly one kilowatt-hour, so the number from part (b) is already the number of units and can be multiplied by the rate straight away.

This is why kWh is used instead of joules for billing. In joules the same energy is 19 800 000 J, which is a clumsy number to price. The three parts together are the whole practical chain: find the current to size the fuse, find the energy in units, price the units.