Answer:
On
Step-by-step explanation:
A preposition can be defined as a word that shows or illustrates the relationship between a pronoun or noun and other words in a sentence. Some examples of a preposition used in various literary works in English language are up, below, after, by, against, for, over, at, to, on, etc.
The main purpose of a preposition as a part of speech is to introduce an object (of, upon), indicate a timeframe (from, by, over), show direction (to, across, along), location or place (at, up, after, below, on) and to illustrate the spatial or sequential relationship between two or more things, people, place, etc.
Hence, the preposition in the sentence "The pirates hid the gold on the island" is on. It illustrates the location where the pirates hid the stolen gold, which is an island.