Ahacker has successfully infected an internet-facing server which he will then use to send junk mail, take part in coordinated attacks, or host junk email content. which sort of trojan infects this server?

Botnet Trojan. Hopefully that’s helpful :)

I’d say platform as a service (paas) or application platform as a service (apaas) or platform base service is a category of cloud computing services that provides a platform allowing customers to develop, run, and manage applications without the complexity of building and maintaining the infrastructure typically associated with developing and launching an app.