The coolant in a nuclear power plant that is most likely to contain boric acid is the primary coolant.

Here's why:

* Boric acid is a neutron absorber: It is used in nuclear reactors as a soluble neutron poison, helping to control the rate of nuclear reactions.

* Primary coolant is directly in contact with the reactor core: The primary coolant circulates through the reactor core, where nuclear fission occurs.

* Secondary coolant typically doesn't need boric acid: The secondary coolant is often used in a steam generator to transfer heat from the primary coolant to produce steam. It's not directly involved in the nuclear reactions and therefore doesn't need boric acid for neutron absorption.

Note: While boric acid is typically added to the primary coolant, its presence may vary depending on the type of reactor and its operating conditions.