The scalar product of two vectors is negative when:

Explanation

(a) They are parallel vectors: When two vectors are parallel, the dot product (scalar product) of the vectors will be positive or zero. The dot product will only be negative if the vectors are anti-parallel, not parallel. (b) They are anti-parallel vectors: When two vectors are anti-parallel, meaning they have opposite directions, the dot product of the vectors will be negative. This is the correct option given. (c) They are perpendicular vectors: When two vectors are perpendicular to each other, the dot product of the vectors will be zero. It is not negative. (d) They are parallel with some magnitude: If two vectors have the same direction but different magnitudes, the dot product will be positive or zero. It will not be negative. So, in summary, the correct option is (b) - the scalar product of two vectors is negative when they are anti-parallel, meaning they have opposite directions.