Vectors possess magnitude and direction

Steps:

• Define a vector as a physical quantity with both magnitude and direction, unlike scalars which have only magnitude.
• Evaluate momentum: it equals mass times velocity, and velocity is a vector.
• Evaluate speed: it is the magnitude of velocity, lacking direction.
• Evaluate temperature and Young's modulus: both are scalar properties without directional components.

Why A is correct:

• Momentum $\vec{p} = m\vec{v}$ is a vector because it inherits direction from velocity $\vec{v}$, per Newton's laws.

Why the others are wrong:

• B: Speed is a scalar, representing only the magnitude of velocity without direction.
• C: Temperature is a scalar thermodynamic property, measured without directional reference.
• D: Young's modulus is a scalar elastic constant, defined as stress over strain (both scalars).

Final answer: A