Apply a test voltage of, say, 60 V (so that the resulting total current will be an integer for either 2 Ω or 3 Ω) across 'a' and 'c' and solve for the currents and voltages in the original circuit. Which answer are they consistent with?
Consider what would happen if you made the 5 Ω resistor 0 Ω. What would the resistance between 'a' and 'c' be? Would that resistance go up or down when we restore the original value of the resistor? What does that tell you about the plausibility of either answer?
This question looks like it was ill posed. The equivalent resistance between 'a' and 'b' is 2 ohms, but the equivalent between 'a' and 'c' is something else other than 2 ohms. Thus you may be right
Often the test voltage is 1v and the current 'i' is measured so that we have:
R=E/i
and since E=1 we have:
R=1/i
Alternately, a test current of 1 amp is used and the voltage 'E' measured so that we end up with:
R=E