Step 1: After the auxiliary verbs "had," "has," and "have," the main verb must be in its past participle form. Step 2: The past participle of "become" is "become" itself, not "became," which is the simple past tense form. Step 3: Since the sentence describes a past perfect situation linked to "was sad," the auxiliary "had" is correct, and it must be followed by "become," giving "had not become." Step 4: Option 2 is the correct correction.