Step 1: Identify the introductory sentence.
Sentence (C) introduces the main topic in a general way: "The vegan diet has gained momentum...". This is a good starting point.
Step 2: Find the next logical step.
After stating that the diet is popular, sentence (D) logically follows by introducing a conflict or "controversy" about it.
Step 3: Look for the sentence that details the controversy.
Sentence (B) elaborates on the specifics of the controversy mentioned in (D). It explains both the positive side (health benefits) and the negative side (restrictive, raises concerns). The phrase "This diet" clearly refers back to the vegan diet.
Step 4: Find the concluding sentence that explains the root cause.
Sentence (A) provides a reason or a conclusion for why the controversy and questions exist: a "Lack of knowledge". This is a fitting final sentence.
Step 5: Assemble the logical order.
The sequence is (C) Topic Introduction \textrightarrow (D) Introduce Controversy \textrightarrow (B) Detail the Controversy \textrightarrow (A) Explain Cause of Controversy. This gives the order (C), (D), (B), (A).