Teaser 2979: Mischievous Sam

From The Sunday Times, 27th October 2019 [link]

I set Sam a question, the answer to which was a 3-digit number, with the digits increasing by 1 from first to last (e.g. 789)

Sam eventually produced a 3-digit answer, but only 2 of his digits were correct and in the correct position. The third digit was wrong.

Investigating further I found that Sam had the correct answer but, for devilment, decided to change it into a different (single-digit) base.

If I were to tell you which of his 3 digits was the wrong one, you should be able to tell me:

(a) the correct answer, and;
(b) the base used by Sam.

What are they?