The form of the word octopus’s is singular possessive. Option A is correct,
The world octopus refers to both, the singular and the plural form.
Since many common nouns end in the letter s, conflicting policies and theories exist regarding how to show possession when writing them.
There are a few who add only an apostrophe to all nouns ending in s.
In case the word in in its plural form, to show plural possession of a itending in an s or s sound, you have to form the plural first; and then immediately use the apostrophe.
