Answer:
a. “When all was ready, the man reached in his pocket for a second piece of birch bark.”
Explanation:
An adverb clause are groups of words that functions as an adverb in a sentence, they can modify a verb, adjective or adverb. They elaborate on when why, how much or where an action in a sentence occur.
It begins with subordinate conjunction like "after", "because", "afterwards", etc. They are group of words that act like an adverb but not have a subject and verb.
Using an adverb clause in a sentence adds better descriptive detail to writing. They can be placed in the beginning, middle or end of a sentence.
From the options, option A contains an adverb clause because of the adverb clause "When all was ready..." as it modified the verb in "the man reached in his pocket.."
