The correct answer is: A. Positive literal
A positive literal is a propositional variable or its negation. It is a literal that is true if the propositional variable is true.
A negative literal is a propositional variable or its negation. It is a literal that is true if the propositional variable is false.
Generalized modus ponens is a rule of inference that states that if $P$ implies $Q$ and $P$ is true, then $Q$ must also be true.
A neutral literal is a propositional variable or its negation. It is a literal that is neither true nor false.
Definite clauses are a type of logical formula that is used in logic programming. They are made up of a head and a body. The head is a propositional variable, and the body is a conjunction of literals.
Positive literals are more suitable normal form to be used with definite clauses because they are easier to understand and manipulate. Negative literals and generalized modus ponens are not as suitable because they are more complex. Neutral literals are not suitable because they are neither true nor false.
Here is an example of a definite clause:
head: X
body: ¬P ⧠Q
This clause states that if $P$ is false and $Q$ is true, then $X$ must also be true.