Answer
4400 ohms
Work Step by Step
Use Ohm’s law. The ac output current (collector) multiplied by the output resistance equals the ac output voltage.
$$V_{out}=i_CR_C$$
We are told the gain of the transistor amplifier and the ac base current, so we can calculate the ac output current.
$$R_C=\frac{V_{out}}{i_C}=\frac{V_{out}}{(\beta)( i_B)}$$
$$ =\frac{0.42V}{95(1.0\times10^{-6}A)}=4421\Omega\approx 4400 \Omega$$
