What is a Service Level Agreement $($SLA$)?$

Select one:

a$.$ A document outlining the scope of work for a particular project

b$.$ A document which describes the broad outlines of an agreement that two or more parties have reached

c$.$ A document which measures the time loss, consequential loss and loss of profits as a result of an$-$interruption to some business critical service or system

d$.$ A document which defines the level of service you expect from a vendor