What is the difference between a hot-water system and a steam-heating system?

Steam radiators and hot water radiators are very similar, so much so that in many cases they are interchangeable. Many homes have been converted from steam to hot water heating systems (and vice versa) without having to change the actual radiators.

The most obvious difference between the two is that a hot water radiator is always equipped with two connections: hot water goes into the radiator on one side, and the cooled water leaves on the other. Steam radiators can have either one or two connectors: hot steam can enter on one side, and condensed water can leave either through the same pipe or through a pipe on the other side.

While it is certainly best to use a steam radiator for steam and a hot water radiator for water, in some cases a switch between the two will require only a few simple operations.

( info from homeadvisor.com)

the correct option is option d