I need to buy the SSL for my client. My client only need to use SSL for abc.clientdomain.com. Not for their domain (client.com). What kind of SSL need to buy for them? Kindly suggest to me.
A standard single domain SSL should do the trick. As long as you do not want to SSL anything else beyond that one subdomain.
Single domain SSLs will secure one common name -- one domain or one subdomain. Once you get into wanting to secure more (domain.com AND subdomain.domain.com) -- that's where a Wildcard SSL comes in.
Here's the What SSL do I need? page.