Why D? And why not the others?

Correct answer: D. They describe the same basic behaviour (their denotation is similar) but carry different emotional associations (their connotation differs) — 'thrifty' sounds like a compliment, while 'stingy' sounds like criticism of that same behaviour.

'Thrifty' and 'stingy' both point to essentially the same underlying behaviour: spending as little money as possible. What separates them isn't the behaviour itself but the attitude the word carries about it — its connotation. 'Thrifty' frames that behaviour positively, as sensible and admirable, while 'stingy' frames the very same behaviour negatively, as mean-spirited or excessive. This is exactly why writers choose one word over the other depending on whether they want to praise or criticise the same action. The claim that 'thrifty' is formal and 'stingy' is slang is false; both are standard, everyday words found in any dictionary. The claim that one describes spending and the other saving is also wrong, since both words are really about the same kind of careful spending, not opposite financial activities. The claim that only 'stingy' extends beyond money is not the real distinguishing feature either — both words can, in fact, be stretched to describe things like praise or effort.

Source: BBC Bitesize KS3 English: Denotation and connotation