What does 'reckon' mean? Is that a British expression?

[val]

Test No. incompl/elem-15 "An Easy Question", question 6

Well, I think it is and I ......... it's an impossible question to answer.

(a) deliver
(b) reckon
(c) report
(d) realize

Test No. incompl/elem-15 "An Easy Question", answer 6

Well, I think it is and I reckon it's an impossible question to answer.

Correct answer: (b) reckon

Your answer was: incorrect
Well, I think it is and I realize it's an impossible question to answer.
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sorry, but i don't understand your explanation, coul you tell me more?

[Torsten]

The person thinks that the question is impossible to answer so reckon is the right word here because it means guess/suppose/assume.
Alan has given a detailed answer to your question here.

TOEFL listening lectures: A lecture in social science class