The bacteria in milk are destroyed when it _______ heated to 80 degree Celsius.

Correct answer: "is".

Explanation: The sentence states a general truth about what happens when milk reaches 80 degrees Celsius. The clause describes the condition of the milk being heated, so the simple present passive is appropriate: "when it is heated to 80 degrees Celsius." Here, "it" refers to the milk.

• Use simple present for general truths.

• Use the passive voice here because the focus is on what happens to the milk/bacteria ("is heated" and "are destroyed").

• Minor wording note: write "80 degrees Celsius" (plural "degrees").