(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.

To arrange the given sentence fragments into a coherent and grammatically correct sentence, let us analyze each part step-by-step:

1. Identify the subject and starting main clause:
Fragment (C) "octopuses have three" provides the primary subject ("octopuses") and main verb ("have"). Thus, (C) naturally serves as the opening part of the sentence.

2. Connect the noun/modifier:
Fragment (C) ends with the numeral "three", which requires a plural noun to complete the phrase. Fragment (D) begins with "hearts, and two", perfectly completing the thought ("three hearts"). So, (C) connects directly to (D).

3. Link the sub-clause/subject:
After "and two" at the end of fragment (D), a prepositional phrase or pronoun is needed to indicate what "two" refers to. Fragment (B) starts with "of them stop beating", which logically fits ("two of them"). Therefore, (D) links directly to (B).

4. Complete the conditional context:
Fragment (A) "when they swim" provides the temporal condition for when two of the hearts stop beating. Placing (A) after (B) completes the meaning of the full sentence.

Final Sequence:
Combining the fragments in the order (C) → (D) → (B) → (A) gives:
"octopuses have three hearts, and two of them stop beating when they swim"

This sentence is grammatically correct and logically sound. Hence, the correct arrangement is option CDBA.