We use an exclamation mark to indicate an exclamatory clause or expression in writing. ‘Yay’ is a commonly used exclamation that indicates excitement. Therefore it should be followed by an exclamation mark. Whereas ‘We are going on a picnic.’ is the next sentence, therefore it should not be followed by an exclamation mark. Hence option A is correct while option B is incorrect. Since ‘We are going on a picnic.’ is a separate sentence, the first letter of the sentence needs to be capitalized. Hence ‘we’ is not the right form of starting a sentence. Thus option C is incorrect. In addition to this, it is a declarative sentence; hence we cannot use a question mark at of it. Therefore option D is incorrect.