Answer:
In Spanish, sentirse (to feel) is the intransitive form of the verb sentir (to feel).
Sentirse is also a pronominal verb, this means the verb needs to be conjugated with a reflexive pronoun, which agrees in person and number with the subject, to express reflexivity and in some cases emphasis.
Here the Reflexive Pronouns according to the subject are:
1st person singular: Yo (I) me
2nd person singular: Tú (You) te
3rd person singular: Él/Ella/eso/esa (He, She, It) se
1st person plural: Nosotros (we) nos
2nd person plural: Ustedes (You) (In latinAmerica) se / Vosotros (In Spain) os
3rd person plural: Ellos/Ellas (They) se
In this sentence the subject is ella (she), which is the 3rd person in singular, and according to the list above, the reflexive pronoun for ella is se.
Therefore, the right answer is:
Ella se siente enferma.