The correct option is
B is
Answer : B
Explanation:
The hint in the bracket tells us that the given statement is a "fact", which means that a truth.
And by rules of grammar; to state a fact we use simple present tense.
And if the sentence is in third person singular, we use to verb form "is" and Option B is the correct answer.
Therefore the correct complete sentence is:
Dennis Heal IS a politician.
"were" is the second person singular past form of "be".
"is being" is used for sentences of present continuous tense, which is not the case here and is again grammatically inappropriate for use here.
"has been" is for sentences of present perfect progressive tense, which also not the case here and thus is also grammatically inappropriate.