How can volcanic eruptions cause the average temperature on the planet to drop?

the ash in the atmosphere blocks the sunlight.

volcanic ash insulates the earth.

volcanic ash is hot.

The answer is A. The ash in the atmosphere blocks the sunlight. 

Can't be c because the temperature dropped so it being hot is irrelevant 
Can't be B because it insulating the earth wouldn't directly cause a drop in temperature.

Plus the ash does in fact, enter the atmosphere and block sunlight as well as cause a variety of dangers. 

Hope this helped. Have a great day!