<u>Answer:</u> The correct answer is Option d.
<u>Explanation:</u>
We are given:
Mass percentage of  = 20 %
 = 20 %
So, mole fraction of  = 0.2
 = 0.2
Mass percentage of  = 30 %
 = 30 %
So, mole fraction of  = 0.3
 = 0.3
Mass percentage of  = 35 %
 = 35 %
So, mole fraction of  = 0.35
 = 0.35
Mass percentage of  = 15 %
 = 15 %
So, mole fraction of  = 0.15
 = 0.15
We know that:
Molar mass of  = 16 g/mol
 = 16 g/mol
Molar mass of  = 28 g/mol
 = 28 g/mol 
Molar mass of  = 26 g/mol
 = 26 g/mol
Molar mass of  = 48 g/mol
 = 48 g/mol
To calculate the average molecular mass of the mixture, we use the equation:

where,
 = mole fractions of i-th species
 = mole fractions of i-th species
 = molar masses of i-th species
 = molar masses of i-th species
 = number of observations
 = number of observations
Putting values in above equation:


Hence, the correct answer is Option d.