Answer:
A
Step-by-step explanation:
First, recognize the appositive phrase in all of the sentences: "who was once a migrant farmer". Appositives are added to provide extra information to the subject, but can be as easily taken out without any harm to the sentence itself.
B gets rid of the personal pronoun "who". Aside from making the sentence choppy, its removal makes it unable to tell who the clause is referring to.
C does not separate the appositive phrase "who was once a migrant farmer" with commas when it should be.
D "whom" is used to refer to the object of a verb or preposition while "who" refers to the subject of the sentence.