The correct answer is A. could.
The driver could not control the bus, therefore the accident took place. This means that the driver did not have the ability to control the bus. This could be due to a number of factors, such as the driver being under the influence of alcohol or drugs, the driver being distracted, or the driver being physically unable to control the bus.
The other options are incorrect because they do not accurately reflect the situation. Option B, had, is incorrect because it implies that the driver had the ability to control the bus but chose not to. Option C, would, is incorrect because it implies that the driver was willing to control the bus but was unable to. Option D, can, is incorrect because it implies that the driver has the ability to control the bus but is not currently doing so.
In conclusion, the correct answer is A. could. The driver could not control the bus, therefore the accident took place.