Which of the following is the proper possessive form for the word "sisters"?
sister's, sisters', sister, none of the above
The proper possessive form for the word "sisters" is "sisters'".
Complete the following task based on the instruction and optional input provided.### Instruction:
Which of the following is the proper possessive form for the word "sisters"?

### Input:
sister's, sisters', sister, none of the above

### Output:
The proper possessive form for the word "sisters" is "sisters'".