Select the most appropriate option to fill in the blank.

The mystery behind the extinction of the dinosaurs ______ obscure.

is still

The most appropriate option to fill in the blank is is still.

The word is is a singular present-tense verb that agrees with the subject mystery, which is singular. In this case, the sentence is saying that the mystery behind the extinction of the dinosaurs is still obscure. This means that the mystery is not yet fully understood.

The other options are not correct.

• Are still does not agree with the subject mystery, which is singular.
• Be still is the infinitive form of the verb be. It cannot be used in this context.
• Being still is the present participle form of the verb be. It cannot be used in this context.