Answer
All three situations have equal brightness.
Work Step by Step
We found that the phase difference is: $~~(a)~ 1.70~~~ (b)~ 1.70~~~ (c)~ 1.30$
Maximum brightness occurs when the waves are exactly in phase, that is, when the phase difference is an integer multiple of $\lambda$.
Since all three cases have a difference of $0.30$ from an integer multiple of $\lambda$, all three situations have equal brightness.
