The correct option is B As a matter of fact, this post office exists in India alone; there are no other floating post offices in the world!
The first letter of the second clause joined with a semicolon is not capitalised except when it is a proper noun or the personal pronoun 'I'. Here, the first word of the second clause i.e. there is neither a proper noun nor a personal pronoun ‘I’. Hence, it is should not be capitalised.