What is the nature of Cas A?

Cas A, or Cassiopeia A, is classified as a Type IIb Supernova remnant, which makes this choice correct. This classification arises from its origin; Cas A is the result of a massive star that exploded at the end of its life cycle. Type II supernovae occur when a massive star exhausts its nuclear fuel, leading to a core collapse that triggers a catastrophic explosion.

The "b" in Type IIb refers to the specific nature of the progenitor star, indicating that it had a brief red supergiant phase. After shedding much of its outer hydrogen envelope, this star underwent the supernova explosion, resulting in the remnant we observe today. Cas A is notable for being one of the youngest supernova remnants in our galaxy, with an estimated age of around 350 years.

The other options describe different astronomical objects or phenomena that do not align with the characteristics of Cas A. A white dwarf is the remnant of a star like our Sun, not a massive star that undergoes a supernova. A black hole is formed from the remnants of a massive star after it loses enough material and can occur after supernova events, but Cas A itself is not a black hole. An open star cluster