Correct option is A. $$\dfrac{b}{b+g}$$
Given number of boys in the class $$=b$$
Number of girls in the class $$=g$$
Total number of students in the class $$=b+g$$
$$\therefore$$ Ratio of boys to total number of students $$=$$ Number of boys: Total students
$$\begin{aligned}&=b:(b+g) \\&=\frac{b}{b+g}\end{aligned}$$