Are you happy with a C?
SEO optimization is important to ensure you can maximize ranking potential and drive traffic to your website from search engines like Google.
Nobody is going to stay on a website they can't use! Usability is important to maximize your available audience and minimize user bounce rates.
40% of users will leave a page if it takes more than 3 seconds to load. Performance is important to ensure good user experience and reduced bounce rates.
Security is important to ensure your website protects user data, doesn't become compromised or experience downtime or data loss.