Cloning of alien DNA by the host involves -

The correct answer is Option (1) → Attaching at the specific sequence site (ori) in the host

A piece of alien DNA cannot copy itself in a host cell. It must be linked to an origin of replication (ori), the sequence from which replication starts; this is why a vector is used.

Option (1) Attaching at the specific sequence site (ori) in the host — Correct. Linked to the ori, the alien DNA is copied along with the vector in the host.

Option (2) Complementing recombinant with non recombinant fragments — Incorrect. Recombinants and non-recombinants are told apart and separated (by antibiotic or colour selection), not joined.

Option (3) Ligating at multiple recognition sites — Incorrect. A vector should have very few, preferably single, recognition sites; several sites would give several fragments and complicate cloning.

Option (4) Insertion activation technique — Incorrect. There is no such technique. The real term is insertional inactivation, a way of selecting recombinants.

NCERT Extract:
Cloning Vectors – …