The complete infinitive phrase in this sentence is 'to learn methods for solving equations', because 'to learn' is the infinitive, but the rest of the phrase also has to be included.
Answer: A) to learn methods for solving equations.
Explanation: An infinitive phrase is a group of words that start with an infinitive form (to+verb), it can have the function of a verb, an adverb or an adjective. An infinitive phrase should include the infinitive form and also all the modifiers and objects that help to make the phrase more descriptive. In the given sentence the infinitive phrase is "to learn methods for solving equations" and it has the function of an adverb.
