A

Step 1: In the initial state, the gas occupies volume V_1 at temperature T.
Step 2: When the partition is removed, the gas expands into the total volume of (V_1 + V_2).
Step 3: Since the container is insulated, no heat is exchanged with the surroundings; therefore, the process is adiabatic.
Step 4: For an ideal gas undergoing free expansion, there is no work done on or by the gas, and no heat is exchanged.
Step 5: Thus, the internal energy of the gas remains constant, which implies that the final temperature remains the same.
Step 6: Thus, the final temperature after the expansion is still T.
Therefore, the final temperature is T.