Unique trait of vectors: direction

Steps:

• Scalar quantities possess only magnitude, like mass or temperature.
• Vector quantities possess magnitude and direction, like velocity or force.
• Both scalar and vector quantities have magnitude and associated units.
• Direction is the sole characteristic exclusive to vectors.

Why A is correct:

• By definition, vectors require direction for complete description, while scalars do not.

Why the others are wrong:

• B: Scalars possess magnitude, so it's not unique to vectors.
• C: Both scalars and vectors possess magnitude and units.
• D: Both scalars and vectors possess units.

Final answer: A