The sentence that is most clearly objective is "My parents first got me a dog when I was 12 years old," as stated in option C and explained below.
A sentence is objective when it is unbiased. That means it should not express an opinion or point of view, by simply state a fact. The speaker's feelings about the topic should not compromise the sentence. Let's analyze each one of the sentences provided in the question:
With the information above in mind, we can conclude that option C is the correct answer.
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