The correct answer is: is.
The verb “is” is used to describe a state of being. In this case, the state of being is that Dennis Heal is a politician. This is a fact, so the present tense verb “is” is the most appropriate choice.
The other options are not as appropriate because they do not accurately describe the state of being. The verb “were” is used to describe a state of being in the past. The verb “is being” is used to describe a state of being that is in progress. The verb “has been” is used to describe a state of being that has been completed.
In conclusion, the correct answer is “is” because it is the most appropriate verb to describe the state of being that Dennis Heal is a politician.