In build-to-suit projects, developers design and construct buildings to meet the needs of a primary tenant, who pays the developer a marginal fee for design and construction work and agrees to buy the completed building or
will occupy all or most of it in exchange for a favorable long-term lease.
The tenant will agree to two things: (1) buy the completed building; OR (2) occupy all or most of it.
These two things need to be represented grammatically parallel to convey that^.
A and B are incorrect because "will occupy" converts the second option into a verb phrase, showing certainty
(A)
will occupy all or most of it
(B)
will occupy all of it, or most of it,
Keeping the infinitives: "to buy OR to occupy" show the clear relationship between the two options the buyer has with the sentence as a whole.
(C)
to occupy all or most of it
D is incorrect for reasons similar to A and B
(D) occupy all of it, or most,
E is incorrect because 'they' does not agree in number with the subject "the primary tenant" (singular)
(E)
they will occupy all of it, or most,