The specific DNA sequence which is responsible for initiation of replication is known as:

Origin of replication (ori)

The correct answer is Option (4) → Origin of replication (ori)    

Origin of replication (ori) is a sequence from where replication starts and any piece of DNA when linked to this sequence can be made to replicate within the host cells. This sequence is also responsible for controlling the copy number of the linked DNA. So, if one wants to recover many copies of the target DNA it should be cloned in a vector whose origin support high copy number.