(c) vector a . vector r.
(d) vector F . vector r.
EXPLANATION:
The dot product is the product of the magnitudes of the vectors and cosine of the angle between them. Only those options will be negative in which the vectors have always opposite directions i.e. the angle between them is π so that cosπ=-1.
F and a has always the same direction, so cos0=1, the option (a) is always positive hence not true.
v and r are sometimes in the same direction and sometimes in opposite direction, so their dot product is not always negative. Hence (b) is not true.
r and a are always in opposite directions hence their dot product will always be negative, hence (c) is true.
F and r are always in opposite directions hence their dot product will always be negative, hence (d) is true.
