The question refers to a circuit diagram and graphical options A–D, but in the provided prompt the actual circuit figure and the option graphs are missing.

Because of that, the quantity $Q_2$ on the $2\,\mu F$ capacitor cannot be determined from first principles here: the relation $Q_2(C)$ depends entirely on how the capacitors and source are connected.

What is missing

To solve this, we would need:

1. The circuit diagram showing the position of the variable capacitor $C$ and the $2\,\mu F$ capacitor.
2. The four graph/options A, B, C, D.

Without the circuit, many different expressions are possible, for example:

• if capacitors are in series, charge may be the same on both and vary as equivalent capacitance,
• if in parallel, the charge on the $2\,\mu F$ capacitor may be constant,
• in bridge-like arrangements, $Q_2$ may vary nonlinearly or even change sign.

So the problem is under-specified in the text provided.

Comparison with stored answer

The stored correct answer is D, but since the essential figure/options are absent, I cannot independently verify that from the given information alone.

If you upload the circuit figure and option graphs, I can solve it fully and verify whether D is correct.