Step 1: Identify the introductory sentence.
Sentence (C) introduces the main topic generally: "The vegan diet has gained momentum...". This is a good start.
Step 2: Find the next logical step.
Following the popularity statement, sentence (D) introduces a conflict or "controversy".
Step 3: Detail the controversy.
Sentence (B) elaborates on the specifics of the controversy in (D). It explains positives (health benefits) and negatives (restrictive, raises concerns). "This diet" refers back to the vegan diet.
Step 4: Find the concluding sentence.
Sentence (A) provides a reason for the controversy: "Lack of knowledge". This is a fitting conclusion.
Step 5: Assemble the logical order.
The sequence is (C) Topic Introduction \textrightarrow (D) Introduce Controversy \textrightarrow (B) Detail Controversy \textrightarrow (A) Explain Cause. This gives the order (C), (D), (B), (A).