Answer
Only (a) is true, and rest-- (b), (c), and (d) --are false.
Work Step by Step
a) is true as:
Mass number of Calcium = 40
Atomic number = 20.
Thus number of protons = Atomic number = 20.
Since the ion has a 2+ ( $Ca^{2+}$) charge, there are two electrons less than number of protons. Thus number of electrons = 20 - 2 = 18.
Hence the statement is true.
b) This statement is false, as creator of the Cathode Ray Tube was William Crookes, and founder of Charge to mass ratio was J.J. Thomson, not Rutherford.
c) Statement is false, as protons are much heavier than electrons.
d) Statement is false, as the nucleus contains nucleons , the term used for protons and neutrons together.
Electrons are not in the nucleus, but are present around the nucleus, revolving around it in orbits.
