What is the specific role of Xist in mammalian X-inactivation?

What is the specific role of Xist in mammalian X-inactivation?
 
  a. It encodes a protein that triggers heterochromatin formation on one X and suppresses it on the other X.
  b. It is a DNA element acting as a binding site for proteins that trigger heterochromatin formation.
  c. It produces an antisense RNA that prevents inactivation of one X chromosome.
  d. It produces an RNA that accumulates on the future inactive X.
  e. It encodes a transcription factor that activates genes producing proteins that function in X-inactivation.