Final answer:
All options are grammatically correct, but 'The cat sat on the mat' is the most clear due to its simple structure. 'The dog chased after the ball', 'The bird flew in the sky', and 'The sun shone brightly' are also clear and grammatically sound.
Step-by-step explanation:
The question of which sentence is most clearly written and grammatically correct is a matter of assessing clarity, conciseness, and grammar. In this case, all four options provided in the student's question are grammatically correct, but sentence A, The cat sat on the mat, is likely the most clearly written due to its simplicity and directness, which removes any possibility for ambiguity or complexity.
Each word in the sentence has a clear function, and there's a straightforward subject-verb-object structure. Similarly, sentence B, The dog chased after the ball, is also clearly written, with the action of the dog being the focal point. Sentence C, The bird flew in the sky, is also correct, detailing the action of the bird with an adverbial phrase 'in the sky' providing the location of the action. Lastly, sentence D, The sun shone brightly, is grammatically sound, with 'brightly' functioning as an adverb describing the manner of the sun's shining.