The isoelectronic set of ions is:

🧠 The Multi-Ion Match We need a set where every single ion has the exact same electron count. This usually involves checking elements across periods 2 and 3 that have reached the $1s^2 2s^2 2p^6$ configuration.

🗺️ The Electronic Audit Checking Option (c):

• $\ce{N^3-}$: $7 + 3 = 10 \text{ e}^-$
• $\ce{O^2-}$: $8 + 2 = 10 \text{ e}^-$
• $\ce{F^-}$: $9 + 1 = 10 \text{ e}^-$
• $\ce{Na^+}$: $11 - 1 = 10 \text{ e}^-$ All members have 10 electrons, making them isoelectronic.

⚡ The Lithium Filter Notice options (a), (b), and (d) all contain $\ce{Li^+}$. Helium's electronic configuration is $1s^2$ ($2 \text{ e}^-$), while Neon's is $1s^2 2s^2 2p^6$ ($10 \text{ e}^-$). Since $\ce{Li^+}$ ($2 \text{ e}^-$) is present alongside species like $\ce{Na^+}$ or $\ce{Mg^2+}$ ($10 \text{ e}^-$), these options crash immediately.

⚠️ Common Traps Don't let the similar atomic numbers of $\ce{Li}$ ($Z=3$) and $\ce{Na}$ ($Z=11$) fool you into thinking their ions are isoelectronic. They are in the same group, but their ions fall back to different Noble Gas cores.

$\boxed{\text{Answer: (c)}}$