A dilute acid solution is prepared by adding the concentrated acid to water. If you do the reverse, i.e. add water to the acid it is possible that a small amount water could react with acid and boil and explode thus splashing the acid onto you or someone else near the activity.
The answer is 546K. If you need the answer in degrees Celsius you subtract 273 from 546 and get 273 degrees Celsius. I included how I got the answer in the photo below. Hope this helps!
H2 has the lowest boiling point.