In a heterogeneous chemical reaction requiring the use of a solid catalyst, the rate of the reaction could not be increased by increasing the concentration of reactants, but was doubled by doubling the amount of solid catalyst added. This probably indicated that

The active sites on the initial amount of catalyst were fully saturated

Explanation:

A catalyst is a substance which alters the rate of a chemical reaction but largely remains unaltered at the end of the reaction. A catalyst has an active site which enables it to catalyze a reaction.

The active site provides the best possible orientation for reactant molecules to collide in order to form products.

When the active sites are not fully saturated with reactant molecules, more reactant molecules can bind and the reaction can proceed much faster. However, if the active sites of the catalysts are fully saturated, the reaction has maximum velocity and cannot go anymore faster.

Addition of more catalyst molecules can however make the reaction proceed faster as a it provides more active sites for reaction for the reactants.

i'm pretty sure its b i might be wrong though!

its first and third is your answer i did the qustion the fourth isnt right