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Solution

Statement (A) is false.

Entailment means if $\alpha$ is true, then $\beta$ must be true in all models where $\alpha$ is true.

Correct definition is:

$\alpha\models\beta$

means $\beta$ is true in every model in which $\alpha$ is true.

But statement (A) says few worlds, so it is false.

Statement (B) is true.

Forward chaining and backward chaining are natural reasoning algorithms for Horn form knowledge bases.

Statement (C) is false.

Sound inference algorithm derives only those sentences that are logically entailed.

But deriving all entailed sentences is the property of completeness, not soundness.

Statement (D) is true.

Propositional logic does not scale well to environments of unbounded size because it cannot represent general rules using variables and quantifiers.

Therefore, true statements are (B) and (D).