Arrange according to the instructions given in bracket:
`Mg^(2+), Cu^(2+), Na^(+), H^(+)` (In the order of preferential discharge at the cathode )

Explanation

To arrange the ions \( Mg^{2+}, Cu^{2+}, Na^{+}, H^{+} \) in the order of preferential discharge at the cathode, we need to consider the electrochemical series, which ranks cations based on their tendency to gain electrons (be reduced) during electrolysis.

### Step-by-Step Solution:

1. **Identify the Ions**: We have four ions to consider: \( Mg^{2+} \), \( Cu^{2+} \), \( Na^{+} \), and \( H^{+} \).

2. **Understand the Cathode Process**: At the cathode, reduction occurs, meaning that positively charged ions (cations) gain electrons. The ion that is more easily reduced will be discharged first.

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