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

In formal English, an abstract noun (such as 'bombast') is defined by another noun or noun phrase, not by a clause beginning with 'when' or 'where'. A 'when' clause specifies a time and a 'where' clause specifies a place; using either to define a quality or practice creates a grammatical mismatch known as faulty predication, even though it is common in casual speech.

The given sentence defines bombast with 'is when high-sounding words ... are used', a time-clause standing in for a noun. The fix must replace this clause with a noun phrase that avoids 'is when'/'is where' and keeps the contrast between using such words for effect and using them for suitability. The correct choice does exactly this: it turns the definition into 'the use of high-sounding words for effect rather than for suitability', a noun phrase that is grammatically parallel to the subject and preserves the full contrast.

• 'is when high sounding words for effect, not suitability, are used' -- repeats the original 'is when' fault; a time-clause still cannot define the noun.

• 'is where high-sounding words for effect rather than for suitability are used' -- replaces 'when' with 'where', but a place-clause has the identical fault of defining an abstract noun via a clause rather than a noun phrase.

• 'is the using of high-sounding words for effect only' -- uses the non-idiomatic 'the using of' instead of 'the use of', and drops the 'suitability' contrast, narrowing the meaning to 'only'.

Only the phrasing built on 'the use of ... rather than for suitability' both fixes the grammatical fault and keeps the original meaning intact.