What is the grammatical error in the sentence: "Long before his death, Pablo Picasso had become one of the most important artist in history."?

The correct answer highlights a verb tense error present in the sentence. The phrase "had become" is the proper past perfect tense that indicates an action completed before another event in the past, which is appropriate here given the context of Picasso’s life and achievements. The incorrect form, "had became," misuses the verb "become," as "became" is the simple past form of the verb. In this context, the use of "become" with "had" is necessary to maintain the grammatical structure of the past perfect tense, allowing the sentence to convey that his importance was established prior to his death.