Ion charges are integer multiples of the elementary charge

Steps:

• Elementary charge e = 1.6 × 10^{-19} C; ion charge q = ±n e, n integer.
• Divide each option by e to check if n is integer.
• A: -4.8 × 10^{-19} / e = -3 (integer).
• B: +1.6 × 10^{-19} / e = +1 (integer); C: +3.2 × 10^{-19} / e = +2 (integer); D: +4.0 × 10^{-19} / e = +2.5 (not integer).

Why D is correct:

• Charges are quantized per the elementary charge law; non-integer multiples are impossible for ions.

Why the others are wrong:

• A: Equals -3e, possible for triply charged anion.
• B: Equals +e, possible for singly charged cation.
• C: Equals +2e, possible for doubly charged cation.

Final answer: D