For formal phone calls, introduction of the caller can be given in the following manner, EXCEPT-

Concept: In informal discussion a person has to maintain graveness of his use of words and hence the choice of words in the judgment is truly vital.

• “ Good morning! ” – Here good morning salutation is used, so this is also an example of a formal phone call.
• “ Could I speak to you, please? ” – Here the speaker formally asks the receiver, so it is also a formal manner.
• “ Hi! What’s up? ” – Using Hi! is not a formal way of talking in a formal call.  Hi! is a popular salutation used when talking with family members and with friends.
• On the other hand ‘ what’s up? ’ is used for a truly informal way of questioning the status of a person.
• “ I am Mr. X calling from, organisation. ” – This type of introduction can also be used as in this introduction speaker formally or gently introducing himself on the call. 
• The discussion that shouldn't be used is C- “ Hi! What’s up? ”

Hence, option C is the correct answer.