Answer:
The sentence that has an error in capitalization is letter B. I already heard the news about Jamie and Alex: they are going to have a baby!
Step-by-step explanation:
In American English, different circumstances determine whether or not to capitalize the first word after a colon. If we are merely listing things and the first word after the colon is not a proper noun, there is no need to capitalize it. The same happens when we have a phrase or incomplete sentence that does not start with a proper noun.
However, when we do have a complete sentence after a colon, even if it does not start with a proper noun, its first word should be capitalized. That is precisely the reason why the sentence in letter B is incorrect. After the colon, we a have a sentence that has a subject, a verb, and an object - a sentence that expresses a complete thought. Thus, the correct capitalization would be:
I already heard the news about Jamie and Alex: They are going to have a baby!