This symbol < – –> indicates:

a. that heat must be applied.

b. that the reaction is reversible.

c. an incomplete combustion reaction.

d. that a gas is formed by the reaction.

B. That the reaction is reversible

The ↔ sign is used in reactions where the equilibrium constant of the forward reaction is close to the equilibrium constant of the backward reaction. This results in the backward reaction being possible. An example of such a reaction is:
N₂ + 3H₂ ↔ 2NH₃