Question 3
Which isotope of hydrogen has two neutrons?
The right answer is the one marked above.
Tritium, written 31H. Protium has 0 neutrons and deuterium has 1.
Read the notation and you need not remember: the bottom number is the atomic number (1 for all three, since all three are hydrogen), and the top is the mass number. Neutrons = A − Z, so tritium is 3 − 1 = 2.