The difference between simple interest and the true discount on Rs.2400 due 4 years hence at 5% per annum simple interest is:

Given,

Rate (R) = 5%

Time (T) = 4 years

Amount = Rs. 2400

True Discount = $$\frac{A\times R\times T}{100+RT}=\frac{2400\times5\times4}{100+\left(5\times4\right)}=\frac{2400\times5\times4}{120}$$ = Rs 400

Simple interest = $$\frac{P\times T\times R}{100}=\frac{2400\times4\times5}{100}$$ = Rs 480

$$\therefore\ $$Difference between Simple interest and True discount = 480 - 400 = Rs 80

Hence, the correct answer is Option D