Answer:
Sales revenue = $220,000
Variable costs = $55,000
Fixed costs = $24,000
∴ Operating Income = Gross Income - Operating Expenses
Operating Income = $220,000 - ( $55,000 + $24,000 )
Operating Income = $141,000
∵ Sales increase by $40,000
∴ Variable cost proportion to sales =

⇒ Variable cost proportion to sales =

⇒ Variable cost proportion to sales = 0.25
Now, the Variable cost would become :
Variable cost(new) = Variable cost(old) + Sales increase × Variable cost proportion to sales
Variable cost(new) = 55000+ 40000 × 0.25
Variable cost(new) = 65000
Fixed Cost will remain the same
∴ Operating Income(new) = Gross Income - Operating Expenses
Operating Income(new) = $260,000 - ( $65,000 + $24,000 )
Operating Income(new) = $171,000
∴Increase in Operating Income is $30,000 ($171,000 - $141,000)
i.e. Option(c) is correct.