I was taught that "We should clean..." = "Pono kākou e ho'oma'ema'e..." The exercise states "E ho'oma'ema'e kākou..." which indicates the command form. The command form for "kākou" is "let us all." This is why I hope moderators are reading these.
That is one of the picky things about the program and English grammar. Let's is a contraction of Let us, thus the hint showing us. The options to choose should be either Let's or Let us but not Let or us only.
If you said "E hoʻomaʻemaʻe ʻoe i ka papahele" or "E hoʻomaʻemaʻe i ka papahele" then it would be "Clean the floor". The "E __ kākou" in this sentence makes it "let us __" because you are also including yourself now.