Which best defines the term empirical evidence?

In the sentence, “studies show that non-exercise activity thermogenesis, or n.e.a.t. — a measure of how much energy people use to stand, fidget, walk to the car and otherwise move around without formally exercising — often (declines) substantially after weight loss, perhaps because the body thinks you are starving and directs you to stay still and conserve energy,” the word declines means a. deteriorates. b. improves. c. ascends. d. increases.