Directions: Select the option that contains a grammatical error in the given sentence.

The rationale behind the sudden postponement was due to the chief auditor falling ill unexpectedly.

The correct option containing a grammatical error is "sudden postponement was due to".

Explanation:
The given sentence is: "The rationale behind the sudden postponement was due to the chief auditor falling ill unexpectedly."

The error in this sentence is one of redundancy (or tautology). The noun phrase "The rationale behind" already signifies the reason or logical basis for an event. Therefore, following it with "was due to" creates unnecessary repetition of the cause-and-effect relationship.

Corrected Sentence Options:
1. "The rationale behind the sudden postponement was the chief auditor falling ill unexpectedly."
2. "The sudden postponement was due to the chief auditor falling ill unexpectedly."