Difference between dialect and accent

Test No. [color=blue]incompl/inter-93 “Hotchpotch”, question 8

He spoke the language in a regional , which made it impossible for me to understand him.

(a) voice
(b) accent
(c) jargon
(d) dialect

Test No. [color=blue]incompl/inter-93 “Hotchpotch”, answer 8

He spoke the language in a regional dialect, which made it impossible for me to understand him.

Correct answer: (d) dialect

Your answer was: [color=red]incorrect
He spoke the language in a regional accent, which made it impossible for me to understand him.
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what would be the difference here If I used accent?

A dialect is usually spoken by people who live in a certain region of a country. Those people speak their mother tongue in their own individual way. For example, many Scottish people have a dialect.
An accent usually describes the way people pronounce words of a language that is different from their mother tongue. For example, Arnold Schwarzenegger speaks English with an Austrian accent.[YSaerTTEW443543]

TOEFL listening discussions: Why is the student talking to the advisor?[YSaerTTEW443543]

You’re right

Why not regional accent. Different state people use the same language with different accent which becomes difficult to understand sometimes.

You make a good point there. People who speak with thick regional accents are hard to understand!
But I think the reason “accent” doesn’t fit in with the sentence is because it needs a different preposition - “with”.

Thanks for a prompt reply.