$[1]4.$ For rotation about a fixed axis the normal component of acceleration that is directed toward the axis of rotation is expressed in terms of the angular velocity $$ and the vector $r$ directed from the axis of rotation to the point of interest as

a$)a_n=(r)$

b$)a_n=r(r)$

c$)a_n=r$

d$)a_n=(r)$