How is the plural of words ending in 'ex', 'ix', or 'yx' generally formed?

The plural of words ending in 'ex', 'ix', or 'yx' is typically formed by replacing the ending with 'ices'. For example, the word 'index' becomes 'indexes' or 'indices', 'matrix' becomes 'matrices', and 'apex' turns into 'apices'. This rule applies because these specific endings often indicate a Latin origin, where the pluralization follows a distinctive pattern that shifts the suffix rather than simply appending additional letters.

While some words might deviate from this rule, such as 'indexes' being a standard pluralization in certain contexts (especially in English), the classical pluralization for these terms remains 'ices'. The other choices represent incorrect methods for forming plurals for this particular category of words.