Answer
(b) 0.75 mm.
Work Step by Step
In a standing wave pattern, the distance between antinodes $d = \lambda/2$.
We know that $v = f\lambda \implies \lambda = v/f = \frac{1540}{10^6} = 1.54 \times 10^{-3} m = 1.54 m$.
Then $d = \lambda/2 = 1.54/2 = 0.77 mm ≈ 0.75 mm,$ which is choice (b).
