In the given question, a sentence is divided into four parts and given in the alternatives (A), (B), (C), and (D). There is an error in one part. Choose the alternative containing that part of the sentence that has the error. Choosing the alternative with the error will be the answer.

A significant advantage with adopting a balanced diet is enhanced mental clarity and focus.

The correct answer is advantage with adopting a balanced diet.

Step 1: Analyze the given sentence
The full sentence provided is: "A significant advantage with adopting a balanced diet is enhanced mental clarity and focus."
We need to examine each segment of the sentence to find the part that contains a grammatical error.

Step 2: Identify the prepositional error
The error is located in the preposition used after the noun "advantage" in the segment "advantage with adopting a balanced diet".
In English grammar, the noun "advantage" is idiomatically paired with the preposition "of" (or sometimes "in") when referring to a positive feature or benefit associated with an action or situation.
Using the preposition "with" in this context is incorrect.

Step 3: Correct the sentence
- Incorrect: "...advantage with adopting..."
- Correct: "...advantage of adopting..."
The correct sentence should read: "A significant advantage of adopting a balanced diet is enhanced mental clarity and focus."

Therefore, the part containing the error is advantage with adopting a balanced diet.