Focal length from lens formula for real image

Steps:

• Identify object distance (u) as distance from lens to object.
• Identify image distance (v) as distance from lens to real image.
• Apply lens formula: 1/f = 1/v + 1/u (converging lens, real image convention).
• Solve for f, which equals distance LX based on diagram positions.

Why B is correct:

• LX matches f from lens formula, as it is the distance where rays converge for the given object-image setup.

Why the others are wrong:

• A: LW is object distance u, not f.
• C: LY is likely image distance v, not f.
• D: LZ is total object-to-image distance, not f.

Final answer: B