A maximum spanning tree is a spanning tree of a weighted graph having maximum weight. It can be computed by negating the weights for each edge and applying Kruskal's algorithm (Pemmaraju and Skiena, 2003, p. 336).
A maximum spanning tree can be found in the Wolfram Language using the command FindSpanningTree[g].
More things to try:
Weisstein, Eric W. "Maximum Spanning Tree." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/MaximumSpanningTree.html