Let's analyze each option:

☀ The unexpected noise had terrified the dog.

This option is in the past perfect tense and represents an action that happened before another past action. It does not accurately represent the original sentence's meaning, which is in the past simple tense.

☀ The unexpected noise terrified the dog.

This option is the correct active form of the given sentence. It maintains the past simple tense and follows the structure where the subject "the unexpected noise" performs the action "terrified" on the object "the dog."

☀ The unexpected noise has terrified the dog.

This option is in the present perfect tense, which does not match the tense of the original sentence.

☀ The unexpected noise terrifies the dog.

This option is in the present simple tense, which does not match the tense of the original sentence. Additionally, the original sentence is in the past tense, so this option is not correct.

Correct answer: ☀ The unexpected noise terrified the dog.