Final answer:
The correct use of 'who' and 'whom' is based on their role in the sentence; 'who' as a subject and 'whom' as an object. The answer to the sentence presented is C. who/whom.
Step-by-step explanation:
Correctly utilizing pronouns such as who and whom depends on their function in the sentence. In the phrase 'It was a Californian by the name of Philip Johnston,', we need a pronoun that serves as the subject of the clause that follows. Therefore, the correct choice would be 'who' because it is used as the subject of the clause 'who had learned the Navajo language'. In the second instance, within the phrase 'with (who/whom) he had grown up', the pronoun serves as the object of the preposition 'with', which means that the correct choice is 'whom'. Therefore, the correct answer to fill in the blanks is C. who/whom.