Concept:The error is a missing definite article before a specific economic event.
Explanation:In part (a), "During recession" is incorrect.
The word "recession" refers to a specific economic downturn.
Therefore, it requires the definite article "the" to be clear.
The correct phrase should be "During the recession".
Part (b) "many companies were compelled" is correct in structure and meaning.
Part (c) "to decrease their workforce" is also correct, using proper verb and noun agreement.
Thus, the error is only in part (a).
Answer:Option (a)