Final answer:
d) Teresa of Avila
The influential nun who founded her own order during the Reformation and was declared a Doctor of the Church is Teresa of Avila.
Step-by-step explanation:
The most influential woman in the Church during the Reformation who established her own order of nuns and was declared a 'Doctor of the Church' in 1970 by Pope Paul VI is Teresa of Avila.
She was a prominent figure during the mid-sixteenth century in Spain, a period concurrent with the Reformation. As part of her contributions, Saint Teresa wrote several books describing her spiritual visions.
Her work and influence in the monastic life were significant, at a time when monasticism allowed women to play a role in society independent of their relationship to a father or husband, similarly to historical figures such as Radegund, who found the religious life preferable to secular challenges.
Recognizing her importance, the title of 'Doctor of the Church' denotes a saint whose writings or preachings are useful to Christians 'in any age of the Church.'