Answer
a. Argon: 18 protons and 18 eletrons
b. Manganese: 25 protons and 25 electrons
c. Iodine: 53 protons and 53 electrons
d. Cadmium: 48 protons and 48 electrons
Work Step by Step
The atomic number ( number of protons) is represented by the number above the elements in the periodic table.
The number of electrons is the same as protons, but with opposite charges.
The atomic number of Argon is 18, which means 18 protons and also 18 electrons
You can find this answer at pages 114 and 115, topic 4.4 'Atomic Number and Mass Number.'