Final answer:
The sentence in question is a declarative sentence because it states an action that was taken and is not asking a question, giving a command, or showing strong emotion.
Step-by-step explanation:
The sentence 'The instructor wiped the whiteboard that was filthy with notes from the previous instructor's lessons.' is a declarative sentence. This type of sentence makes a statement and ends with a period. Since the sentence provides information about an action that took place and does not ask a question, give a command, or express strong emotion, it fits the definition of a declarative sentence.