Answer:
Simple subject - fear
Complete subject - great fear for the people living in the Keys and Miami
Step-by-step explanation:
The subject of a sentence is what the sentence is telling us about - the person, place, thing, or idea doing something or being described.
The simple subject is the main word or words that the sentence is telling us about.
The complete subject includes all of the words that the sentence is telling us about (including all modifiers).
The main word in the subject is fear. That's why that is the simple subject. The words that surround it (great, for the people living in the Keys and Miami) are the modifiers that additionally describe it (what the fear is like, fear for whom?). That's why the complete subject is great fear for the people living in the Keys and Miami.