An n.p.n transistor with current gain β = 100 in common emitter...

We know that, Voltage gain$= \dfrac{Outputvoltage}{Inputvoltage}$

Then, ${V}_{out} = {V}_{in} \times$voltage gain.

${V}_{out} = {V}_{in} \times$Current gain$\times$resistance gain

${V}_{out} = {V}_{in} \times \beta \times \dfrac{{R}_{out}}{{R}_{in}}$

${V}_{out} = \dfrac{100 \times 10 \times {10}^{3}}{{10}^{3}} \times {10}^{- 3}$

$= 1V$