The correct option is B Simple Sentence
A simple sentence is made up of only one clause, with one subject and
one verb. The given sentence contains one clause, it has the subject "I" and the verb "have been assigned". The direct object is "a special task" and the infinitive phrase "to gather information... ." is the indirect object. Hence it is a simple sentence and option B is correct. A compound sentence and a complex sentence requires at least two clauses, hence options A and C are incorrect. Option D is incorrect because the answer lies in B.