In a series circuit, all of the components are connected in the same 'loop' and the current only has one direction/path it can flow through.
In the first three options, the current has multiple paths it can go through. So these three circuits are parallel and not series.
In the last option, the current only has one path where it can flow through, so that circuit is in series.
So Circuit D is a series circuit.
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Answer
Circuit D
