Answer:
Nathalie connaît la mère de Sarah.
Explanation: Let's work this out step by step.
A and B are conjugations of the verb 'savoir', which means to know how to. Therefore using savoir here would be inappropriate here.
C and D are conjugations of the verb 'connaître' which also means to know, but you use it to describe someone you're familiar with. So they would be appropriate verbs to use in this context.
Let's conjugate the verb 'connaître':
je connais
tu connais
il/elle/on connaît
nous connaissons
vous connaissez
ils/elles connaissent
But we want the 'she' version of the verb, as Nathalie is doing the action, so the correct answer is elle connaît