The correct answer is: 1998.

Why 1998 is correct

Puerto Rico held a major political status referendum on December 13, 1998 , in which voters were asked to choose among several options regarding the island’s relationship with the United States—statehood, different sovereignty formulas, and “none of the above.”

The “none of the above” option won with about 50.5% of the vote, and many commonwealth supporters used that choice specifically to signal support for continuing the existing relationship (the territorial/commonwealth status) rather than changing it.

So, among the options you listed — 1852, 1998, 1952, 1905 — the last referendum in which Puerto Ricans expressed endorsement (via their votes) for maintaining the current relationship with the U.S. is 1998.

Final choice: 1998 ✅

Information gathered from public forums or data available on the internet and portrayed here.