The equilibrium in the system is a dynamic equilibrium.
B
At equilibrium, the rate of the forward reaction (CH4 decomposing into C2H2 and H2) is equal to the rate of the reverse reaction (C2H2 and H2 reacting to form CH4).
C
At equilibrium, the concentrations of all substances are not changing.
D
At equilibrium, both the forward and reverse reactions are still occurring.