Answer:
Boron
Step-by-step explanation:
(a)
H means hydrogen
The atomic mass of hydrogen is 1 and the mass number is 2.
Be means beryllium
The atomic number is 4 and the mass number is 9.
n mean neutron, atomic number is 0 and mass number 1.
By conserving the charge,
the atomic number of X = 1 + 4 - 0 = 5
the mass of X = 2 + 9 - 1 = 10
The element having atomic number 5 and mass number 10 is Boron (B).