The question is the following: "If you want to use the present tense, which word would best fill the blank?". Unfortunately, there is no blank and I think that a part of the sentence is missing. Anyway ,we can definitely exclude the first two options: a. were b. was because they are in past tense. Now we're left with: d. are c. is The choice between them will depend on whether the sentence has a subject in singular, such as "bird" - then we use "is" or in plural, such as "birds" - then we use "are"