Interference with 180° phase difference Steps:

• Path difference Δ = |YZ - XZ| = 110 cm - 70 cm = 40 cm.
• For maximum intensity with 180° initial phase difference, Δ = (2m - 1)λ/2, where m is a positive integer.
• Rearrange: λ = 2Δ / (2m - 1) = 80 / (2m - 1) cm.
• Test integer m: m=3 gives λ = 80/5 = 16 cm, matching option B.

Why B is correct:

• For λ=16 cm, Δ=40 cm = 5(λ/2), an odd multiple of λ/2, satisfying constructive interference condition per phase difference formula.

Why the others are wrong:

• A: λ=10 cm gives 2m-1=8 (even), impossible as 2m-1 must be odd.
• C: λ=20 cm gives 2m-1=4 (even), violates odd multiple requirement.
• D: λ=40 cm gives 2m-1=2 (even), does not satisfy interference condition.

Final answer: B