Assertion : The v-t graph perpendicular to time axis is not...

Assertion is correct: A v-t graph perpendicular to the time axis means that for a single instant of time, there are infinite values of velocity, which is physically impossible. This implies an instantaneous, infinite change in velocity. Reason is correct: Infinite acceleration means an instantaneous change in velocity. Since acceleration (=$\Delta \mathrm{v}$/ $\Delta \mathrm{t}$), if ($\Delta \mathrm{t}$) approaches zero (as in a vertical line on a v-t graph), and ($\Delta \mathrm{v}$) is finite, then acceleration approaches infinity, which cannot be realized in practice. The reason correctly explains why the assertion is true.