Option C is the correct answer because we use the definite article 'the' before a known or specific noun. Here, since we already know the guide is pointing to the pile of sticks (specific noun in question) which he had already discussed the previous day.
Option A is incorrect because 'an' is an indefinite article that comes before a nonspecified noun that starts with a vowel sound. Since the noun in the question is neither nonspecific and nor does it start with a vowel sound, this option is incorrect.
Option B is incorrect because 'a' is only used before a nonspecified singular noun that starts with a consonant sound while the noun in the question is specific.
Option D is incorrect as the sentence needs an article to modify the noun that is present, for the sentence to have a complete meaning.