Work by force equals force times displacement parallel to it

Steps:

• Force F acts parallel to the slope.
• Block displaces distance l along the slope, in F's direction.
• Work by F uses displacement component parallel to F, which is l.
• Thus, work = F × l.

Why B is correct:

• Definition of work: W = F d cosθ; θ=0° (parallel), d=l, so W = F l.

Why the others are wrong:

• A: F h uses vertical height h, but displacement parallel to F is l, not h.
• C: W d involves block's weight W and undefined d, ignoring applied force F.
• D: W l multiplies weight by distance, but work by F depends on F, not W.

Final answer: B