Smallest possible alkane with ethyl group as substituent possess molecular mass:

Ethyl group cannot be a substituent in ethane, propane, or butane. Methyl groups are a possible substitute for them. Methyl and ethyl are incompatible substituents for methane. Therefore, the smallest alkane having an ethyl group as a substituent will be pentane. 

The general form for the smallest possible alkane with a substituent ethyl group. 3-ethyl pentane $\left({\mathrm{C}}_7{\mathrm{H}}_6\right)$ will be the smallest possible alkane with an ethyl group as a substituent.
Its molecular mass will be
${\mathrm{C}}_7{\mathrm{H}}_{16}\to \left(12\times 7\right)+\left(16\times 1\right)$
$=84+16$
$=100\mathrm{u}$
Thus, the molecular mass is 100g
Remember that, even in pentane, 2-ethyl pentane is not possible (incorrect according to IUPAC nomenclature) although the mass will remain the same but the parent carbon chain will then have 6 carbon atoms and the correct IUPAC name will be 3-methyl hexane.

Hence, the correct option is (D).