This sentence is correct as it is written because it makes use of the correct verb tense have been.' It also contains the idiom 'believe to' in order to explain that scientist believes that malaria was once believed to have been introduced by 16th-century European explorers.

Option A - correct
This answer choice uses the correct verb tense which is the present perfect tense 'have been' and the correct idiom 'believed to.'

Option B - incorrect
This option is incorrect states that the present progressive form of the verb 'as being' adds an incorrect notion to the sentence. Furthermore, it explains that 16th-century people are currently introducing malaria. In addition, one time means numerically just one time and not other times. The sentence calls for the present perfect tense, and the idiom structure believed.... as is incorrect.

Option C - incorrect
The use of the present tense verb 'be' is not correct. The former belief in this sentence concerns an occurrence in the past, therefore, the sentence calls for the present perfect tense.

Option D - incorrect
This option incorrectly states believed as having been. The sentence calls for the present perfect tense, and the idiom structure believed. Furthermore, it connotes the same meaning as option B.

Option E - incorrect
this incorrectly stated believed as being. However, the sentence calls for the present perfect tense and the idiom structure 'believed.'