Which one of the following is a secondary alkyl halide?
A.\[\;{\left( {C{H_3}} \right)_3}CCl\]
B.\[\;{C_2}{H_5}Cl\]
C.\[\;{\left( {C{H_3}} \right)_2}CHCl \]
D.\[\;{\left( {C{H_3}} \right)_2}CHC{H_2}Cl \]

Hint: In order to answer the question, let us know about alkyl halides. These are also called haloalkanes which means one or more hydrogen atoms which are present in alkane are replaced by any halogen atom. These are further classified into primary, secondary and tertiary depending on the position where the halogen atom is being attached.

Complete answer:
As I mentioned the further classification, we shall discuss it. The primary alkyl halide is the one where the carbon that is attached to the halogen is bonded with only one alkyl group.
Ex: \[C{H_3} - C{H_2} - Cl\]
In secondary alkyl halide, the carbon to which halogen is attached is bonded with two alkyl groups.
Ex: \[
  C{H_3} - CH - C{H_3} \\
  \;\;\;\;\;\;\;\;\;\;\;|\; \\
  \;\;\;\;\;\;\;\;\;\;Br \\
\]
In tertiary alkyl halide, the carbon to which halogen is attached is bonded with three alkyl groups.
Ex: \[
    \\
  \;\;\;\;\;\;\;\;C{H_3} \\
  \,\;\;\;\;\;\;\;\;\;|\;\;\, \\
  C{H_3} - C - C{H_3} \\
  \;\;\;\;\;\;\;\;\;\;| \\
  \;\;\;\;\;\;\;\;\;Cl \\
\]
Based on the above explanation when you draw the structures of compounds given in the options, the third option which is 2-chloropropane has the structure of secondary alkyl halide because the carbon to which the halogen is attached is bonded with two other alkyl groups and therefore it is said to be secondary alkyl halide. Let us see its structure.
     \[
  C{H_3} - CH - C{H_3} \\
  \;\;\;\;\;\;\;\;\;\;\;| \\
  \;\;\;\;\;\;\;\;\;Cl \\
 \]

Hence, C is said to be the correct option.

Note:
The above classification is based on the position of halogen atoms. There is another classification of alkyl halides on the basis of the number of halogen atoms present in their structure. These are divided as mono haloalkane, dihaloalkane and dihaloalkanes.