The answer is (d): the sound level will increase to 96 dB, as seen by clicking your mouse on the photograph below.
When you use the same source, the two speakers are coherent (in phase), so the total amplitude is twice that of either one and the total intensity is multiplied by 4 (the square of the amplitude). This gives a six-decibel increase, whereas the meter actually reads a bit above 5, which is much closer to 6 than to 3!