The correct answer is C.
OBJ-3.9: A wildcard certificate is a public key certificate that can be used with multiple subdomains of a domain. This saves money and reduces the management burden of managing multiple certificates, one for each subdomain. A single wildcard certificate for *.diontraining.com will secure all these domains (www.diontraining.com, mail.diontraining.com, ftp.diontraining.com, etc.). The other options provided are not solved by using a wildcard certificate.