Answer:
The answer is Then.
Explanation:
The "to prove" part of the information is the THEN of the "if-then" statement.
If is the question like, it is not validated. You are asking something if it holds true.
For example: If it rains heavily, then water bodies fill up.
So, here you are proving that water bodies fill if rains are heavy.
Another example: If a = 3 and b = 6 then a+b = 9
We can see that 3+6=9
Then it is proved that a + b = 9