What was the problem with the farm tools?

Of the answers you've been given to choose from, I'd have to say the fact they "were made for human hands". Another example would be as follows;

But the problem the animals could not at first solve was how to break up the stone into pieces of suitable size. There seemed no way of doing this except with picks and crowbars, which no animal could use, because no animal could stand on his hind legs.