Since 2007, the Department of Defense has maintained a program designe : Sentence Correction (SC)

Since 2007, the Department of Defense has maintained a program designed to investigate unidentified flying objects, despite the views of many scientists who considered that these phenomena will be found to have natural explanations.

consider means to give attention or thinking about a possibility which is incorrect here as per the meaning of the sentence.
Option A and B are out.

scientists who believe that these phenomena will be.... is more apt and clear in meaning.
Option D is correct.

C. despite the views of many scientists believe
Incorrect. scientists who believe is correct rather than scientist believe that...

E. although scientists believed - past tense is incorrect.

Answer D