The correct answer is: Notional whole units which represent incomplete work.

Equivalent units are notional whole units which represent incomplete work and which are used to apportion costs between work in process and completed output.

The quantity of work achievable in one hour at standard levels of performance is the description of a standard hour.

A unit of output which is identical to others previously manufactured in the same process is not the definition of an equivalent unit. All completed units in process costing are identical.

A unit of output in relation to which costs may be ascertained is the description of a cost unit.