Question

Deuterium is the most common isotope of hydrogen. How many neutrons does it have?
(A) $4$
(B) $3$
(C) $2$
(D) $1$
(E) $0$

Answer 1

Hint
Mass number is the sum of neutrons and protons present in a nucleus. We know that the atomic number gives us the number of electrons and protons. From subtracting the atomic number from the mass number, we get the number of neutrons in an atom. Isotopes are elements of the same atomic number and different mass numbers.
$\Rightarrow N = M - A$
Here, The atomic number is represented by $A$, Mass number is represented by $M$, and the number of neutrons is represented by $A$

Complete step by step answer
Deuterium is an isotope of hydrogen, which means the atomic number or the number of protons and electrons are the same as that of a hydrogen atom. The atomic number of Deuterium is $1$. The mass number of Deuterium is $2$.
Subtracting the atomic number from the mass number we get the number of neutrons.
$\Rightarrow N = M - A $
$\Rightarrow N = 2 - 1 $
$\Rightarrow N = 1 $
The number of neutrons in a Deuterium is $1$
Hence option (D) $1$ is the correct answer.

Additional Information
Isotopes are variants of a particular chemical element that differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom. For example, carbon-12, carbon-13, and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13, and 14, respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons so that the neutron numbers of these isotopes are 6, 7, and 8 respectively.

Note
Hydrogen is the only element whose isotopes have different names in common use today: the ${}^2H$ (or hydrogen-2) isotope is deuterium and the ${}^3H$ (or hydrogen-3) isotope is tritium. The symbols D and T are sometimes used for deuterium and tritium.