The correct option is
B alone
Let's read the passage in order to fill the blank. We can understand that the lion couldn't kill the bulls when they were united. But one day, when the bulls had a fight, they separated and they went in different directions alone to graze. This was the time when the lion killed the bulls. Let's look at the options.
Option A - When the bulls were united, the lion couldn't kill them, as they all formed a ring and hit the lion with their horns. Hence, option A is incorrect.
Option C - The passage states that the bulls went in the different directions to graze, it doesn't say that they were grazing. They went alone in different directions and hence, the lion could kill them. Hence, option C is incorrect.
Option D - The passage doesn't mention that the bulls were running when the lion killed them. Hence, option D is incorrect.
Option B - As the bulls had a fight, all the four bulls went in different directions alone. That's when the lion could kill them. Hence, we can say - 'The lion killed all the bulls when they were alone.' Thus, option B is correct.