Q34,paper3,J 13.Horn clauses are special kinds of propositions which can be described as

(A) Single atomic proposition on left side.

(B) Single or multiple atomic proposition on left side.

(C) A single atomic proposition on left side and a single atomic proposition on right side.

(D) A single atomic proposition on left side or an empty left side.

Answer D.

explanation.

A literal is an atomic formula or its negation.

A clause is a disjunction of literals

A Horn clause is a clause with exactly one positive literal

A Horn formula is a conjunctive normal form formula whose clauses are all Horn 

Example.

VX ( human(X) → mortal(X) ).

VX(~human(x) v mortal(x)).