How many structural isomers are possible if one hydrogen in diphenylmethane is replaced by chlorine?

The correct option is 4.

To find the number of structural isomers possible when one hydrogen atom in diphenylmethane is replaced by a chlorine atom (monochlorination), we need to analyze the structure of diphenylmethane and identify the number of chemically non-equivalent (unique) hydrogen positions available for substitution.

Diphenylmethane has the molecular formula $C_{13}{H}_{12}$ and consists of two benzene rings connected by a central methylene ($-{\mathrm{CH}}_2-$) group. Its structural formula can be represented as:
$C_6{H}_5-{\mathrm{CH}}_2-C_6{H}_5$

Due to the symmetry of the molecule, the two phenyl rings ($-C_6{H}_5$) are equivalent. We can classify the hydrogen atoms in diphenylmethane into distinct positions based on their chemical environment:

1. Methylene Position (Aliphatic carbon):
There is one central carbon atom ($-{\mathrm{CH}}_2-$) bridging the two rings. Replacing one hydrogen here gives the first isomer:
${\mathrm{(C}}_6{H}_5{)}_2\mathrm{CHCl}$ (Chlorodiphenylmethane)

2. Ortho-Position (Aromatic ring):
On each phenyl ring, the carbons adjacent to the attachment point (the carbon connected to the $-{\mathrm{CH}}_2-$ group) are the ortho positions. Because of symmetry, all four ortho positions across the two rings are chemically equivalent. Replacing one hydrogen at any of these ortho positions gives the second isomer:
(2-chlorophenyl)(phenyl)methane

3. Meta-Position (Aromatic ring):
The carbons that are two bonds away from the attachment point on the rings are the meta positions. All four meta positions across the two rings are chemically equivalent. Replacing one hydrogen at any of these meta positions gives the third isomer:
(3-chlorophenyl)(phenyl)methane

4. Para-Position (Aromatic ring):
The carbons directly opposite to the attachment point on the rings are the para positions. Both para positions (one on each ring) are chemically equivalent. Replacing one hydrogen at either of these para positions gives the fourth isomer:
(4-chlorophenyl)(phenyl)methane

Since there are exactly 4 distinct hydrogen environments in diphenylmethane (1 on the methylene bridge and 3 on the aromatic rings: ortho, meta, and para), replacing one hydrogen with chlorine yields exactly 4 structural isomers.