Determining Cost Basis w JTWROS

[Jim Kay]

Hoping for some tax guidance on cost basis determination. Many thanks in advance.

My wife was listed on her's Mom's house deed as joint tenet with right of survivor-ship. Her Mom passed away in Nov '17 and the house was worth $225,000 at that time.

We are now looking into selling the house and trying to determine our cost basis. The house was built for $150,000 and her Mom paid the entire amount. Understanding that we can add in improvements that have been made over the years but trying to confirm our starting point. Is it;

 A) $150,000 + improvements

 B) Would a half step up in basis apply? Would this be calculated by taking half the original basis plus half the new basis? $75,000 + $112,500 = $187,500 + improvements

 C) Would a full step up in basis apply? Her Mom paid for the entire house so with her contribution being 100%, would the new basis be the value at the time of her death? $225,000 

Again, many thanks for any help provided.