Vector quantities have magnitude and direction

Steps:

• Define vector quantity as one with both magnitude and direction, unlike scalars with only magnitude.
• Examine each option: density measures mass per volume (scalar); displacement measures change in position (vector); energy measures work capacity (scalar); speed measures rate of motion (scalar).
• Identify displacement as the only vector among the choices.

Why B is correct:

• Displacement is a vector, defined as the straight-line distance from initial to final position with direction, per the vector displacement formula $\vec{d} = \vec{r_f} - \vec{r_i}$.

Why the others are wrong:

• A: Density is a scalar, calculated as mass/volume without direction.
• C: Energy is a scalar, conserved in systems without directional component.
• D: Speed is a scalar, the magnitude of velocity without direction.

Final answer: B