How does nutritionist and weight loss expert Jackie Keller explain why some individuals who go vegan gain weight, and what advice does she offer to help those who become vegan avoid gaining weight?

a) Jackie Keller explains that going vegan can lead to weight gain due to an increase in calorie-dense vegan foods, and she advises individuals to focus on whole, plant-based foods and portion control.
b) Jackie Keller suggests that going vegan can cause weight gain because of the body's adjustment to new dietary patterns, and she recommends regular exercise and mindful eating practices.
c) Jackie Keller believes that vegan diets inherently lead to weight gain, and she advises individuals to avoid vegan diets altogether.
d) Jackie Keller attributes weight gain in vegans to lack of protein and essential nutrients, and she suggests incorporating vegan protein sources and supplements into the diet.