Metabolic pathways can be controlled through the availability of enzymes and modulation of their catalytic activities. Enzyme availability can be regulated via , while enzyme activity is commonly regulated via .

a) substrate and cofactors b) covalent modification

Explanation:

Metabolic pathways involve all the chemical processes takes place in the enviornment or in an organisms.The metabolic pathways are controlled through the catalytic activities of enzymes.

The availability of enzyme is regulated  by substrate and cofactors present in enzyme and enzyme activity is commonly regulated via covalent modification.

Enzymes are highly selective in nature and bind to a specific substarte only. The active site in enzyme binds with the substrate to form enzyme substrate complex. Coactors assist the enzyme activity, without cofactor enzyme can not perform its activity.

Hence enzyme availibility is regulated by  substrate and cofactors regulates.

Covalent modifications regulates activity of enzyme as it involves addition and removal of chemical group to synthesis required protein. It can change the chemical properties of the site by targeting one or multiple amino acid.

Thus the correct answer is a) substrate and cofactors b) covalent modification