Final answer:
In 'Pudd'nhead Wilson', the only method to tell Thomas and Valet apart is through their fingerprints. The book utilizes fingerprint analysis at a time when other methods like DNA testing were not available. DNA fingerprints match a child's genetic markers with those of their parents to establish relatedness.
Step-by-step explanation:
The only way to tell Thomas and Valet apart in Mark Twain's novel Pudd'nhead Wilson is B. Fingerprints. In the novel, Pudd'nhead Wilson, a lawyer and amateur sleuth, uses his interest in fingerprinting to resolve the central mystery of the book—the identities of two characters, Tom and Chambers (Valet), who were switched at birth. While birthmarks, mannerisms, and DNA tests can all be used to distinguish between individuals, in the context of this 19th-century story, fingerprint analysis is the only method available and employed by the characters.
To answer a specific question related to the use of DNA: For a child's DNA fingerprint to establish a relation to the mother and father, it must contain a combination of genetic markers that are inherited from both parents. These markers are often short sequences of DNA known as short tandem repeats (STRs). The child's DNA will have STRs that match the mother at certain locations or loci and match the father at others.