A. city-states ruled by kings
This one is correct. There's an iconic artifact called "The Priest King" to support this.
B. Monotheism
This one is false. It is known that the Indus Valley civilization had many gods. Most likely some sort of combination of Hinduism, Buddhism and Jainism.
C. writing system not yet deciphered
This one is also correct. Even though we know a lot more, we are not even close to having deciphered everything.
D. signature seals used in trade
This is also correct. According to historians, those seals are evidence of settlements of Indus valley traders in ancient Mesopotamia.
A, B and D are correct