Core Idea - Complete Graph

A complete graph is a graph where every pair of nodes has an edge. With $n$ nodes, you have $\frac{n(n-1)}{2}$ edges. For this problem, generating all edges upfront takes $O(n^2)$ time and space. That is acceptable for small $n$ (up to $1000$).

Kruskal still works, but you will spend time sorting $O(n^2)$ edges. Prim might be faster here because it does not require generating all edges upfront. Think about the tradeoff.