Among the following which is not a horn clause?

• p

• Øp V q

• p → q

• p → Øq

...

Horn clause

A Horn clause is a proposition in propositional logic that contain at most one positive literal. For example a propositional letter preceded by a positive sign and an arbitrary number of negative literals i.e propositional letters preceded by a negation sign.

Here p → Øq is not a Horn clause because it contain one positive literal (p) and one negative literal (Øq).
So it does not follow the condition of the above definition. You can see below example of a horn clause.

A simple example of a Horn clause is:

P → Q
where P and Q are propositional letters. This clause states that if P is true, then Q must also be true.