The correct option is A a
'A' is an article which is used to refer to something indefinite. 'An' works on the same grounds, but only for nouns/pronouns beginning with a vowel. Something that has already been referred to, indicating that it is definite or specific, would have 'the' as the preceding article. Since the question clearly states that the pond is being referred to for the first time and the succeeding word does not begin with a vowel, 'an' and 'the' may be ruled out. Therefore, 'a' is the correct answer to this question.