Consider the following statements regarding source criticism in historical research:

1. External criticism involves examining the authenticity of a document by studying its material properties, such as paper, ink, and seals.
2. Internal criticism focuses on the content of the document to assess its credibility and inaccuracy.
3. Textual criticism compares different versions of a manuscript to identify changes or errors made by scribes.
4. Comparative criticism ensures that the historical narrative is consistent across various sources.

Which of the above statements are correct?
a) 1, 2, and 3
b) 2, 3, and 4
c) 1 and 3
d) 1, 2, and 4