Correct Answer: Unexpectedly

Solution : The correct choice is the second option.

The idiom out of the blue means unexpectedly or suddenly and it is used to describe something that happens without warning or preparation. When something happens out of the blue it means it comes as a surprise or shock, much like something appearing suddenly from a clear, blue sky. There is no prior indication or warning.

The meanings of the other options are as follows:

• Undoubtedly means without any doubt.
• Unbelievably means in a manner that is hard to believe.
• Unconcerned means without any concern.