If a pilot goes out when the main burner comes on, where is the problem most likely found?

When a pilot goes out simultaneously with the main burner igniting, the issue is most commonly associated with the pressure regulator vent. The pressure regulator's primary function is to ensure the gas pressure remains within safe operating limits for the equipment. If there is a blockage or malfunction in the vent of the pressure regulator, it can cause irregularities in gas pressure, potentially leading to situations where the pilot flame is extinguished when the main burner is activated due to inadequate gas flow or improper pressure conditions. This scenario underlines the importance of the pressure regulator's performance in maintaining stable operation of both the pilot and main burners.

The other options may involve other types of issues related to gas systems, but they are less likely to cause the pilot to extinguish specifically upon the main burner operation. For example, while problems within the pilot burner assembly could lead to a pilot going out, it typically wouldn't be triggered solely by the main burner ignition.