Which of the following cannot be used to obtain a fix?

To understand why buoys cannot be used to obtain a fix, it’s important to look at what a "fix" entails in navigation. A fix refers to a position determined by taking readings from multiple reference points. While buoys are indeed useful navigational aids, they serve primarily as points of reference rather than standalone tools for determining a precise position in conjunction with other data.

Landmarks provide visual points that can be triangulated when combined with other bearings, GPS offers exact coordinates based on satellite data, and charting methods utilize maps to plot a position using latitude and longitude. However, buoys are typically assigned specific coordinates but do not inherently provide a method of triangulation. They help in confirming a position or providing navigational assists, but they lack the comprehensive utility necessary to independently obtain a fix.

In summary, while buoys can assist with navigation, they cannot be directly used to acquire a precise fix without supplemental information. This distinguishes them from the other methods listed, which are all integral to achieving a reliable and accurate fix.