What is the difference between monatomic and polyatomic ions?
A) \[A{l^{3 + }}\] is monoatomic and \[O{H^ - }\] is polyatomic
B) \[NaCl\] is monoatomic ion
C) \[A{l^{3 + }}\] is polyatomic and \[O{H^ - }\] is monoatomic
D) None of the above

Hint: We need to know that it is easy to identify monatomic and polyatomic ions by looking at them. Firstly ions must have a charge on it either positive or negative, they cannot be neutral and secondly these ions can be differentiated on the number of atoms present. No chemical bond is involved in monatomic ions whereas atoms are held together by covalent bonds in polyatomic ions.

Complete answer:
Let’s start this question by knowing about monatomic and polyatomic ion in brief:
The species that is made up of only a single atom per ion is known as monatomic ion whereas polyatomic ions are composed of many atoms per ion.
Monatomic ion is formed when an electron is lost or gained from a valence shell whereas polyatomic ions are formed between several atoms that are held together by covalent or any other forces of attraction.
No chemical bond is involved in monatomic ions whereas atoms are held together by covalent bonds in polyatomic ions.
Examples of monatomic ions are: \[A{l^{3 + }}\], \[N{a^ + }\]
Examples of polyatomic ions are: \[O{H^ - }\], \[N{O_3}^ - \]
The only similarity between these two ions is that they both carry charges which can be positive and negative.
Option A) this option is correct as \[A{l^{3 + }}\] is monoatomic ion as it is made up of only \[Al\] atoms and \[O{H^ - }\] is polyatomic ion as it is made up of \[O\] and \[H\] atoms that are held together by covalent bonds.
Option B) This option is incorrect as \[NaCl\] is a neutral compound which is made up of two monovalent ions \[N{a^ + }\] and \[C{l^ - }\] that together forms \[NaCl\].
Option C) this is an incorrect option as explained above.
Option D) this is an incorrect option as we get the correct option as option A.

Note:
We have to remember that the \[N{O_3}^ - \] is a nitrate ion which is a polyvalent ion as it is made up of $1$ nitrogen and $3$ oxygen atoms. Also it has a negative charge on it which makes it ion and not a neutral compound. We need to know that the \[NaCl\] is a neutral molecule that is formed by ionic bonds between \[N{a^ + }\] and \[C{l^ - }\]. Polyatomic ions are: \[O{H^ - }\] , \[N{O_3}^ - \].