Carol Eats 800 Kcal a Day to Lose Weight

Carol eats 800 kcal a day to lose weight. She is 40 years old. Carol has been walking briskly 60 minutes daily for the past 2 months. When she was younger, she lifted weights, but she has lost considerable lean body mass over the years due to inactivity. Recently, Carol was diagnosed with hypothyroidism which lowers metabolism. Which of the following factors increases Carol's energy output and promotes weight loss?

A) Her 800-kcal diet
B) Her age
C) Her loss of lean body mass
D) Her walking routine