The blackout curtains .

The question asks to complete the sentence about blackout curtains.

• A) sealed light from inside homes: This is the primary function of blackout curtains, to block light.
• B) were used during World War II: Blackouts were a common civil defense measure during World War II to protect against air raids, and blackout curtains were essential for this.
• C) were meant to help keep people safe: By preventing light from being visible from the outside, blackout curtains helped make areas less identifiable to enemy aircraft, thus contributing to public safety.

Since all three statements (A, B, and C) are true regarding blackout curtains, especially in their historical context during wartime, the correct option is D.

The final answer is $\boxed{\text{D}}$