Which is the insect that transmits rocky mountain spotted fever?

A tick. Many patients do not have a history of a tick bite especially children.
Certain ticks... Such as the dog tick and the wood tick are dermacentor ticks, common vectors of rmsf. These ticks are different from ixodes ticks that transmit lyme disease. For the most part, the female tick must feed for at least 24 hours for transmission of an infection to occur.
A tick. The "american wood tick", "rocky mountain wood tick", and "brown dog tick" are the ticks that spread rmsf in the United States. Some people get rmsf but do not remember getting a tick bite. A person with symptoms of rmsf who has been in areas where ticks live, but does not recall a tick bite, may be started on antibiotic treatment while awaiting blood tests to confirm rmsf infection.
Arachnids! Technically "insects" do not transmit the bacteria (rickettsia rickettsii) resposible for rocky mountain spotted fever (rmsf). Ticks - which are technically related to spiders rather than insects - do transmit r. Rickettsi. The most common ticks are dog ticks. However a variety of ticks from around north & south america are capable of spreading rmsf including, although rarely, deer ticks.