Which of the following have the same electron arrangement?

Answer:

2) C

In order to answer the question, we only need to find out how many electrons each of the species have in their atom.

The (₁₇ Cl⁻) has 17 protons. Since it has a charge of (-1), it must have one more electron than proton.
Therefore it has 18 electrons.

The (₁₇ Cl⁺) has 17 protons. Since it has a charge of (+1), it must have one more proton than electron.
Therefore it has 16 electrons.

The (₁₈ Ar) has 18 protons. Since it has no charge, it must have equal number of electron and proton.

Therefore it has 18 electrons.

The (₁₉ K⁺) has 19 protons. Since it has a charge of (+1), it must have one more proton than electron.
Therefore it has 18 electrons.

The (₂₀ Ca⁺) has 20 protons. Since it has a charge of (+1), it must have one more proton than electron.
Therefore it has 19 electrons.

The (₁₉ K⁻) has 19 protons. Since it has a charge of (-1), it must have one more electron than proton.
Therefore it has 20 electrons.

The only species which have the same electronic arrangement are (₁₇ Cl⁻), (₁₈ Ar) and  (₁₉ K⁺) [all having 18 electrons]

Hence C is the answer.