Which of the following is NOT included in the personal auto policy's definition of covered auto? $($Assume there are no endorsements attached to the policy.$)$

a$-$ car the insured rents while his regular car is in the shop for repairs

b$-$ car the insured purchases during the policy period that does not replace any car already insured under the policy

c$-$ all$-$terrain vehicle owned by the insured

d$-$ small trailer owned by the insured that can be used with the insured's pickup truck