Select the word or phrase that makes this sentence grammatically correct: Theo was in great shape; he all the way back to the pier.

The correct answer is 'swam.' In the sentence, the past tense of the verb 'swim' is needed to maintain grammatical correctness. 'Swam' is the correct past tense form of 'swim,' making the sentence coherent and accurately describing Theo's actions. Using 'swimmed' (Choice B) is grammatically incorrect as 'swim' follows a regular conjugation pattern where the past tense is formed by adding '-ed' to the base form. 'Swum' (Choice C) is the past participle form and is not suitable for this context. 'Swim' (Choice D) is the base form and does not fit the past tense structure required in the sentence. Therefore, 'swam' (Choice A) is the only correct option that fits the sentence grammatically.