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The correct plural form of "index" is "indices." This form is derived from Latin, where many scholarly and scientific terms have retained their original Latin pluralization rules. In Latin, nouns that end in "-ex" typically form their plural by changing the "-ex" to "-ices." This is common in academic and professional contexts, particularly in fields such as mathematics, economics, and science, where the term "indices" is frequently used to refer to lists or collections of data points or items.

While "indexes" is also an accepted plural form, largely due to its adoption in modern English usage and its applicability in a broader range of contexts (like computing and statistics), "indices" maintains a stronger connection to the term’s Latin roots. The other options presented—“indeces” and “indexae”—do not reflect standard or recognized plural forms in English or derived from Latin, making them less appropriate selections.