In the question below, four words marked (A), (B), (C), and (D) are given, which may not be arranged in the correct order. Choose the pair of words that should be interchanged to make the sentence grammatically correct and meaningful.

In ancient times, community bards would orally recite (A) the kingdom's ensure (B) to heritage (C) that historical events were never forgotten (D).

The correct option is Only (B)-(C).

Explanation:
Let us analyze the position of each highlighted word in the given sentence:
- (A) recite: The verb 'recite' fits logically after the modal verb 'would' and adverb 'orally' (i.e., 'orally recite').
- (B) ensure: 'Ensure' is an infinitive verb, but it is placed after the possessive noun 'kingdom's', where a noun is required.
- (C) heritage: 'Heritage' is a noun, but it is placed after 'to', where a base verb is required to form the infinitive 'to ensure'.
- (D) forgotten: 'Forgotten' is correctly placed as a past participle in the passive construction 'were never forgotten'.

Interchanging word (B) ensure with word (C) heritage restores the logical and grammatical correctness of the sentence:
"In ancient times, community bards would orally recite (A) the kingdom's heritage (C) to ensure (B) that historical events were never forgotten (D)."

Thus, the correct pair to interchange is Only (B)-(C).