Exponential function represents as x^2 = x * x or 2^3 = 2*2*2 etc.
As per options,
(A) We are unable to write the number 79 as a exponent.
B) We are unbale to write the number 742 as a exponent as well.
(c) We are unable to write the number 73 as a exponent as well.
(d) We are able to write the number 79.79 as exponent (79)^2. Base is 79 and exponent is 2 and it is single exponential expression as base is only one number.
