Right answer:
a. trabajando
This is the correct answer because it is the more logical option. This sentence is given in the present progressive that we use when talking about something that is happening now. This is formed by the verb estar plus a present participle following the formula:
estar + present participle
The present participle is formed using the ending ndo that matches the ing in English. So está is the conjugation of the verb estar for the third person singular in the simple present and trabajando is the present participle that comes from the verb trabajar. Finally:
Ella siempre está trabajando a la hora del almuerzo.
