Resistance (R) of a resistor is given by Ohm's law as,V= IR
R= V/I
Where,
Potential difference, V= 12 V
Current in the circuit, I= 2.5 mA = 2.5 x 10-3 A
\(R=\frac{12}{2.5\times 10^{-3}}=4.8\times 10^3=4.8 K\Omega\)
Therefore, the resistance of the resistor is 4.8 kΩ