Why _____ her medicine for the past three days? has Nancy been not taking has Nancy not been taking has not Nancy been taking

The correct form of the sentence is "Why has Nancy not been taking her medicine for the past three days?"

Here's why:

1. In English, when forming a question in the present perfect continuous tense, the structure is: Why + has/have + subject + been + verb-ing.

2. The word "not" is used to make the sentence negative. In this case, "not" should be placed after "has" and before "been".

3. Therefore, the correct sentence is "Why has Nancy not been taking her medicine for the past three days?"

The other two options ("Why has Nancy been not taking her medicine for the past three days?" and "Why has not Nancy been taking her medicine for the past three days?") are grammatically incorrect.