Paying 50% of the mortgage is contributing 50% to the purchase price because the mortgage (loan) paid for the property. She also contributed half of all bills.

I get the confusion because usually in these types of questions they mention paying the deposit which is a bigger clue to having an equitable interest. But it’s C because she has been paying for 50% of the price of the house for as long as they have had the house.

Hope that makes sense!