Trusts don't live in a primary residence.  They're a legal fiction.  Are you doing a return for an estate, a revocable living trust, or something else?

Revocable living trust,, Grantor dies in 2020.

Was residence up to her death. Sold by trust.

Beleive it qualifies for exlcusion  121-1-(c)(3)(i)  and furhter research support this treatment.

Thanks for reply.

If it was sold after her death, you would get stepped-up basis and it's unlikely there would be any gain after selling expenses.  

There is no "-1" to Code Section 121, and there's no (c)(3).  Maybe you're looking at Regulations.