A second-order reaction:

a. is the rate-limiting step of a reaction.

b. only occurs in multistep process.

c. has a rate constant with units of (time) - 1.

d. is characterized by two molecules coming together to form a product.

e. occurs when one substrate is converted into one product.

Option d is the correct answer.

Second order reaction is the chemical reaction which depends upon the concentration of two reactants.

Second order reaction is given by:

A + B→P (Two different reactants A and B combine to form product P).

Thus, according to the definition, second order reaction is characterised by two molecules combine together to form product.