In the following circuit, a voltmeter $V$ is connected across a lamp $L$ . What change would occur in voltmeter reading if the resistance $R$ is reduced

Here, the emitter-base junction of the N-P-N transistor is forward biased with battery $V_{BB}$ through resistance $R$ . When the value of $R$ is reduced, then the base current $i_b$ will increase. As a result, the collector current will also increase. $(i_c=\beta i_b)$. Due to the increase in $i_c$, the potential difference across $L$ increases and hence the reading of voltmeter will increase.