how many neutrons does the isotope of hydrogen called deuterium (₂¹h) have?\na three\nb one\nc two\nd four

Explanation:

Step1: Recall isotope notation meaning

In the isotope notation ${Z}^{A}X$, $A$ is the mass - number and $Z$ is the atomic number. For deuterium ${1}^{2}H$, $A = 2$ and $Z=1$.

Step2: Use the formula for number of neutrons

The number of neutrons $N$ is given by $N=A - Z$. Substituting $A = 2$ and $Z = 1$ into the formula, we get $N=2 - 1$.

Step3: Calculate the result

$2-1 = 1$.

Answer:

B. one