What does the term "platform$-$as$-$a$-$service $($PaaS$)"$ refer to in cloud computing?

All of these are equally correct.

Cloud services that handle software upgrades and security patching

Cloud services that host websites and blogs

Cloud services that provide an on$-$demand environment for developing, testing, and managing software applications

Cloud services that supply IT infrastructure on a pay$-$as$-$you$-$go basis