Bombast is when high sounding words for effect, not suitability, are used.

CONCEPT: In English, a noun is properly defined by stating what it IS -- a category or action -- not by stating WHEN or WHERE it occurs. Constructions like "X is when..." or "X is where..." are widely considered non-standard for definitions, because "when" and "where" clauses answer questions of time and place, not identity. The accepted pattern for a formal definition is a noun/gerund phrase: "X is the [noun] of...".

APPLICATION: Bombast describes high-sounding words used for effect rather than for suitability. Reading the correct definition in place -- "Bombast is the use of high-sounding words for effect rather than for suitability" -- shows a standard noun-phrase definition ("is the use of...") that keeps the full effect-versus-suitability contrast intact.

CONTRAST -- why each other option fails:

• "is when high-sounding words... are used" defines Bombast via a TIME clause ("when"), a recognised usage error for definitions.

• "is where high-sounding words are used..." defines Bombast via a PLACE clause ("where"), the same category of error applied to location.

• "is the using of high-sounding words for effect only" uses the non-standard gerund "the using of" (rather than "the use of") and drops the essential contrast with suitability, replacing "rather than for suitability" with merely "only".

CROSS-CHECK: Reading each option back into the full sentence confirms this: only the noun-phrase version reads as a natural, complete dictionary-style definition; the time-clause and place-clause versions read as grammatically awkward, and the "the using of...only" version loses the intended for-effect-rather-than-suitability contrast.