Answer:
Cobalt has one less proton.
Explanation:
Nickel and Cobalt both has almost same atomic mass number approximately 59 but has different atomic number.
Number of proton and electrons in an atom is given by,
Number of proton = Number of electron = atomic number of an atom.
Number of neutron in an atom is given by,
Number of neutrons = Atomic mass number - atomic number
Case 1: Cobalt
Atomic mass number = 59
Atomic number = 27
Number of proton and electrons in Nickel is given by,
Number of proton = Number of electron = atomic number of an atom.
Number of proton = 27
Number of proton = 27
Number of neutron in an atom is given by,
Number of neutrons = Atomic mass number - atomic number
Number of neutrons = 59 - 27
Number of neutrons = 32
Case 2: Nickel
Atomic mass number = 59
Atomic number = 28
Number of proton and electrons in Nickel is given by,
Number of proton = Number of electron = atomic number of an atom.
Number of proton = 28
Number of neutron in an atom is given by,
Number of neutrons = Atomic mass number - atomic number
Number of neutrons = 59 - 28
Number of neutrons = 31
Cobalt has one less proton.
Thus, Option D is correct.