The correct answer is: D. no article
The reason is that the word “horseback” is a compound noun, which means that it is made up of two words that are joined together to form a new word. In this case, the two words are “horse” and “back.” When a compound noun is made up of two common nouns, it does not take an article.
For example, we would not say “He goes there on a horse.” We would simply say “He goes
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