You are more likely to have good mental and emotional health

a.
if you are younger than 50
b.
if you're never by yourself
c.
if you have a strong support system
d.
if you have a high-paying job

You are more likely to have good mental and emotional health C. if you have a strong support system.
Although having a high-paying job may be cool, it may also be quite stressful on your health. A is incorrect because there are many older people who have great mental and emotional health. B is just stupid.

The best choice for you to go with will be option C) because with a strong support system it keeps you mentally and emotionally stronger.