The mole fraction of benzene in a solution containing $39\%$ by mass in an organic solvent of molecular mass $122$ is :

The correct option is A $0.6$
The mole fraction of benzene in a solution containing 39% by mass in an organic solvent of molecular mass 122 is :

$\frac{n_b}{n_b+n_o}=\frac{61/78}{\frac{61}{78}+\frac{39}{122}}=0.6$

Here b represents benzene and o represents organic solvent.
The molar mass of benzene is 78 g/mol.

The percent by mass of the organic solvent is $100-39=61$ %

Hence the correct option is B.