Step 1: Concept
An isothermal process occurs at constant temperature. For an ideal gas, this is governed by Boyle's Law: $P_1V_1 = P_2V_2$.
Step 2: Meaning
Boyle's Law states that pressure and volume are inversely proportional when temperature is held constant ($P \propto 1/V$).
Step 3: Analysis
If the pressure is doubled ($P_2 = 2P_1$), then according to the relation $V_2 = (P_1/P_2)V_1$, we get $V_2 = (P_1/2P_1)V_1 = V_1/2$.
Step 4: Conclusion
Therefore, the volume of the gas will be reduced to half of its initial value.
Final Answer: (B)