The wikipedia page for Tseitin transformation includes a nice example of the transform itself. In the example, it is implied that there is no need to create a fresh variable for a single literal (that is not negated).
What is the Tseitin transform of a single literal, e.g. A?
Is it just $A$?
Or is it $X_1 \land A$?
Or is it $X_1 \land (X_1∨¬A)∧(¬X_1∨A)$?