Which layer defines an interface that applications can use to request network services, rather than referring directly to applications themselves?

William travels a lot on business purpose. he needs to regularly communicate with his business partner. he also needs to send out weekly reports to his boss while he is traveling. which web-based application best suits william’s needs? (social media, webmail, wiki) is the best web-based application for william. he can access this application via the internet using a (digital cable, fax machine, web browser).