The correct option is B There are two boys. The sentence should read "boys' rucksacks".
Apostrophe is used to indicate belonging. Apostrophe is used before 's' with singular nouns and after 's' with plural nouns. The given sentence mentions two boys, therefore the apostrophe is placed incorrectly. It should be placed after 's' in 'boys' to show that the rucksacks belong to both the boys. Therefore, the correct option is: A) There are two boys. The sentence should read "boys' rucksacks".