Isoelectronic species are atoms, ions, or molecules that possess the same total number of electrons. To find the correct option, we compare the electron counts:

1. Target Ion (Be²⁺): Beryllium (Atomic number $Z=4$) loses 2 electrons. Total electrons = $4 - 2 = 2$.
2. Option Analysis:

• H⁺: Hydrogen ($Z=1$) loses 1 electron. Total = $1 - 1 = 0$ electrons.
• Li⁺: Lithium ($Z=3$) loses 1 electron. Total = $3 - 1 = 2$ electrons. (Match found)
• Na⁺: Sodium ($Z=11$) loses 1 electron. Total = $11 - 1 = 10$ electrons.
• Mg²⁺: Magnesium ($Z=12$) loses 2 electrons. Total = $12 - 2 = 10$ electrons.

Since both Be²⁺ and Li⁺ have 2 electrons (helium core configuration $1s^2$), they are isoelectronic.