Concept:The error is the missing article before "great deal".
Explanation:In part (a), "great deal" is a singular countable noun phrase.
A singular countable noun must have an article before it.
Because "great" starts with a consonant sound, the article "a" is needed.
The correct phrase is "a great deal".
Therefore, part (a) has an error.
Parts (b) and (c) are grammatically correct.
No error in (b), (c), or the entire sentence.
Answer:A. (a)