The correct answer is 'nació'.
This is because 'nacimos' is the 'nosotros' indicative preterite form of the verb 'nacer' (to be born), which means "we were born".
'Naciste' is the 'tú' preterite form of 'nacer', which means "you were born", but it's informal. Sandra is talking to her grandfather (abuelo), and refers to him with 'usted', so it can't be 'naciste' either.
'Nací' is the 'yo' preterite form of 'nacer', which means "I was born", but she is referring to her grandfather being born in 1926, so it can't be 'nací' either.
