What is the mass of a girl who weighs 850 N?

The quantity of matter present in a body is referred to as its mass. M represents it, and the SI unit of mass is kg.The force with which the earth pulls any object placed on it is referred to as the object's weight. It is represented by the symbol W, and the SI unit of weight is N. 
$\mathrm{Weight} \left(\mathrm{W}\right) = \mathrm{Mass} \left(\mathrm{m}\right)\times \mathrm{Acceleration} \mathrm{due} \mathrm{to} \mathrm{gravity} \left(\mathrm{g}\right)$

According to the question given that,

Weight of the girl, W = 850 N

Thus, 

$$\begin{gathered} \mathrm{W} = 850 \mathrm{N} = \mathrm{m}\times 9.8 \mathrm{m}/{\mathrm{s}}^2 \\ \mathrm{m} = \frac{850 \mathrm{kg}.\mathrm{m}/{\mathrm{s}}^2}{9.8 \mathrm{m}/{\mathrm{s}}^2}=86.73 \mathrm{kg} \end{gathered}$$

Hence, the correct option is B.