The correct answer is: A. 3, 4 and 2
In a broad-band decoupled 13C NMR spectrum, each carbon atom will produce a single signal, with the chemical shift being determined by the electronegativity of the atoms bonded to the carbon. In ortho-dichlorobenzene, the three carbon atoms are all equivalent, so they will produce a single signal. In meta-dichlorobenzene, the two carbon atoms in the middle are equivalent, while the two carbon atoms on the ends are different. This means that there will be two signals in the spectrum, one for the two equivalent carbon atoms and one for the two different carbon atoms. In para-dichlorobenzene, all four carbon atoms are equivalent, so they will produce a single signal.
Here is a diagram of the three isomers of dichlorobenzene, with the carbon atoms labeled:
In ortho-dichlorobenzene, the three carbon atoms are all equivalent, so they will produce a single signal. In meta-dichlorobenzene, the two carbon atoms in the middle are equivalent, while the two carbon atoms on the ends are different. This means that there will be two signals in the spectrum, one for the two equivalent carbon atoms and one for the two different carbon atoms. In para-dichlorobenzene, all four carbon atoms are equivalent, so they will produce a single signal.
Here is a table showing the number of signals expected in the broad-band decoupled 13C NMR spectrum of each isomer of dichlorobenzene:
Isomer | Number of signals
——- | ——–
Ortho-dichlorobenzene | 3
Meta-dichlorobenzene | 2
Para-dichlorobenzene | 1