Final answer:
The underlined portion of the sentence is an adverb clause that modifies the verb 'is born.'
Step-by-step explanation:
The underlined portion of the sentence, "As sure as sparks fly upward," is an adverb clause. Adverb clauses modify verbs, adjectives, or adverbs and answer questions such as how, when, where, why, and to what extent. In this case, the adverb clause modifies the verb "is born," explaining that mankind is born to trouble.