Which one of the following is a planar molecule
A. \[NH_3\]
B. \[H_3O^{+}\]
C. \[BCl_3\]
D. \[PCl_3\]

Hint: Trigonal planar structure is depicted by a molecule having one central atom bound to the other three atoms resembling the corner of an equilateral triangle. The bond angles are \[120^o\].

Complete step by step solution:Here in this question, we have to find out which of the following molecules has a planar structure.

A. \[NH_3\]
Here N is the central atom. Nitrogen has five electrons in its valence shell.
Three electrons are involved in bond-making and two other electrons pair up.
So, it has three bond pairs and one lone pair.
So, it has a trigonal pyramidal structure.
So, A is incorrect.

B. \[H_3O^{+}\]
Here O is the central atom. It has a positive charge and thus has five electrons in this valence shell.
Three electrons bond with three electrons each from a hydrogen atom. One lone pair is present.
So, there are three bond pairs and one lone pair.
So, it has a trigonal pyramidal structure.
So, B is incorrect.


C. \[BCl_3\]
Here Boron is the central atom.
It includes three electrons in its valence shell.
Three electrons are involved in bond-making.
So, it has three bond pairs.
So, it has a trigonal planar structure.


Image: Structure of BCl3
So, C is correct.


D. \[PCl_3\]
Here P is the central atom. It includes five electrons in its valence shell.
Three electrons are involved in bond-making and two other electrons pair up.
So, it has three bond pairs and one lone pair.
So, it has a trigonal pyramidal structure.
So, D is incorrect.

So, option C is correct.

Note: In chemistry, hydronium is the common name for the aqueous cation H3O+.
It is usually considered as the positive ion present when an Arrhenius acid undergoes dissolution in water.
Arrhenius acid molecules in solution lose proton \[H^+\] to the surrounding water molecules forming hydronium ions.