(A) when they swim
(B) of them stop beating
(C) octopuses have three
(D) hearts, and two

The following question gives a sentence which has been divided into four parts which may not be in their correct order of narrative. The sentence is then followed by five options, one of which gives the sequence of the rearranged parts to make the sentence meaningful. Choose the option which gives the correct sequence of the parts. If the sentence is already arranged in the correct sequence, mark option " No rearrangement required " as the answer.

The correct option is CDBA.

Step-by-step breakdown of the sentence rearrangement:

1. Identify the subject/opening part: Part (C) "octopuses have three" sets up the subject ("octopuses") and introduces the subject-verb structure of the sentence. Therefore, part C is the ideal opening fragment.

2. Find the logical continuation: Part (C) ends with "three", which requires a noun to complete the phrase. Part (D) begins with "hearts, and two", creating the clause "octopuses have three hearts, and two". Thus, D follows C directly (CD).

3. Connect the next clause: Part (D) ends with "and two", which is referring back to the remaining hearts. Part (B) provides the verb phrase "of them stop beating", making the sequence "and two of them stop beating". Thus, B follows D (CDB).

4. Complete the thought: Part (A) "when they swim" provides the conditional time context for when two of the hearts stop beating. Thus, part A completes the sentence logically (CDBA).

Complete Meaningful Sentence:
"Octopuses have three hearts, and two of them stop beating when they swim."

Hence, the correct sequence is CDBA.