Etiquette Q: Job Interview Thank You Note, Forms of Adress Therein

In My Humble Opinion (IMHO)
#1

Ok, so I had two interviews at a small law firm for a summer job. I want to send thank you notes to the partners who interviewed me for the position.

The firm has 2 partners, a man and a woman. They are married (they told me) and the woman uses her husband’s last name. When addressing the card for the woman, which is more appropriate?:
A. Mrs. X
B. Ms. X

Where X is the surname both share.

I’m leaning towards Ms. X. What say you all?

#2

Ms. Firstname X, definitely. Unless she had something different on her name plate or letterhead.

Regards,
Shodan

#3

Yup, Ms.

#4

I agree, you can’t go wrong with Ms.