Final answer:
Denial is the most likely defense mechanism responsible for the patient not returning for her follow-up appointment after discovering a breast tumor, which aligns with the first stage in Kübler-Ross's model of grief. The correct option is D.
Step-by-step explanation:
The defense mechanism most likely contributing to the patient's actions of not returning for a follow-up appointment after discovering a breast tumor, especially given her mother's recent death from the same disease, is denial. This is a psychological response where the individual refuses to accept the reality of a distressing situation.
Elizabeth Kübler-Ross described denial as the first stage in the five-stage model of grief, wherein a person is unwilling to accept that they are facing a severe illness, often thinking 'This can't be happening to me' or 'I feel fine.'