Hello!
The answer is:
The empirical formula is the option B. ![NH_{3}](/tpl/images/0446/3582/5e30f.png)
Why?
The empirical formula of a compound is the simplest formula that can be written. On the opposite, the molecular formula involves a variant of the same compound, but it can be also simplified to an empirical formula.
![MolecularFormula=n(EmpiricalFormula)](/tpl/images/0446/3582/01b13.png)
We are looking for a formula that cannot be simplified by dividing the number of molecules/atoms that conforms the compound.
Let's discard option by option in order to find which formula is an empirical formula (cannot be simplified)
A. ![N_{2}O_{4}](/tpl/images/0446/3582/70572.png)
It's not an empirical formula, it's a molecular formula since it can be obtained by multiplying the empirical formula of the same compound.
![N_{2}O_{4}=2(NO_{2})](/tpl/images/0446/3582/76865.png)
B. ![NH_{3}](/tpl/images/0446/3582/5e30f.png)
It's an empirical formula since it cannot be obtained by the multiplication of a whole number and the simplest formula. It's the simplest formula that we can find of the compound.
C. ![C_{3}H_{6}](/tpl/images/0446/3582/c991a.png)
It's not an empirical formula, it's a molecular formula since it can be obtained by multiplying the empirical formula of the same compound.
![C_{3}H_{6}=3(CH_{2})](/tpl/images/0446/3582/3333f.png)
D. ![P_{4}O_{10}](/tpl/images/0446/3582/304c4.png)
It's not an empirical formula, it's a molecular formula since it can be obtained by multiplying the empirical formula of the same compound.
![P_{4}O_{10}=2(P_{2}O_{5})](/tpl/images/0446/3582/4accc.png)
Hence, the empirical formula is the option B. ![NH_{3}](/tpl/images/0446/3582/5e30f.png)
Have a nice day!