it has to be B
kindlt confirm the OA

there are two types of relative clauses: essential/defining (where the modifier is giving essential information and/or defining a particular thing/person) or non-essential/ not defining (where the modifier is giving extra information)

Essential clauses should never be set of by commas and "that" is generally used not "which"
Non-essential clauses should always be set off by commas and "which" is generally used not "that"

Examples:

Essential - The dog that bit me had rabies
Non- Essential - The GMAT, which costs $250, is offered worldwide.

https://www.beatthegmat.com/when-to-use- ... tml#285700

imskpwr wrote:Carnivorous mammals can endure what would otherwise be lethal levels of body heat because they have a heat-exchange network which kept the brain from getting too hot.

(A) which kept
(B) that keeps
(C) which has kept
(D) that has been keeping
(E) having kept

To express a GENERAL TRUTH, we use the SIMPLE PRESENT TENSE.
A general truth is a statement of fact not tied to any particular moment in time: something that is true NOW and is expected to be true ALWAYS.
The SC above is expressing a general truth about carnivorous mammals.
Only in B is the verb in the simple present tense:
They have a heat-exchange network that KEEPS the brain from getting too hot.
Eliminate A, C, D and E.

The correct answer is B.