Step 1: Understanding the Concept:
A compound sentence is formed by joining two or more independent clauses (complete thoughts) using coordinating conjunctions (FANBOYS), conjunctive adverbs (therefore, still), or correlative conjunctions (either...or).
Step 2: Detailed Explanation:
1. Sentence (A): "Her fear is that she may fall ill."
The part "that she may fall ill" is a dependent noun clause. Since it contains one independent and one dependent clause, it is a complex sentence.
2. Sentence (B): "Sheela is clever, still she does not do well..."
This joins two independent clauses using "still" (adversative). It is a compound sentence.
3. Sentence (C): "Noel was caught stealing, therefore he was punished."
This joins two independent clauses using "therefore" (illative). It is a compound sentence.
4. Sentence (D): "That animal is either a fish or a serpent."
Using correlative conjunctions to provide alternative choices is considered a compound structure in traditional grammar.
Step 3: Final Answer:
Sentences (B), (C), and (D) follow the rules for compound sentences, while (A) is a complex sentence.