Let $v$ be the speed of sound in air, $v_L$ velocity of observer at time t. As the observer approaches the source, therefore, apparent frequency

$f=\frac{(v+v_L)}{v}{f}_0=\left[\frac{v+(0+at)}{v}\right]f_0=f_0+\left(\frac{f_{0}at}{v}\right)$

This is the equation of a straight line with a positive intercept $\left(f_0\right)$and positive slope $\left(\frac{f_{0}a}{v}\right)$. Therefore, option (d) is correct.