In exclamatory sentences, we typically begin with "How" to express surprise, emphasis, or strong feeling. In this sentence, the speaker is expressing surprise or emphasis about how cool the morning was yesterday.

Option (1), "How cool yesterday morning was!" follows the structure "How + adjective + subject + verb," and it correctly conveys the exclamatory tone and the emphasis on the coolness of the morning. The other options do not construct the sentence in the appropriate exclamatory form or do not convey the intended emphasis.