Unsure of answer. have just my name on the mortgage as of Oct 2020 and my former spouse hasn't helped at all with payments.

I received Form 1098. I was asked " Do any of these uncommon situations apply to your loan with Computershare Holdings, Inc. d/b/a Specialized Loan Servicing LLC?", which is my mortgage company. I'm unsure what the correct answer would be. Since the mortgage has been in my ex spouse's name also until Oct. 2020 and I'm the only one paying on the mortgage how would I answer this? "Someone co-owns this loan, and I only pay a portion of it" is the closest to the answer I think I should choose. or None of these apply. Please help.

If your former spouse did not pay any of the mortgage payments, meaning you paid 100% of them, then you would be entitled to deduct 100% of the interest.  I would indicate "None of these apply".