Reactions that undergo shifts in their equilibrium must be... A. reversible reactions
B. chemical reactions
C. physical reactions
D. nuclear reactions

A. Reversible reactions.

Explanation:

Reactions that undergo shifts in their equilibrium must be reversible reactions.

A reversible reaction is a chemical reaction that is capable of forming it's reactants back from the resulting formation of products. This simply means that, reversible reactions are chemical reactions that are in equilibrium because the forward and reverse path happens at the same rate.

For example, the reaction of hydrogen gas and iodine gas to form a chemical compound called hydrogen Iodide is a reversible chemical reaction.

Forward reaction:

Reverse reaction:

A reversible reaction is denoted with a double arrow.

Also, reversible chemical reactions are controlled by the Le Chatelier's principle.

c (diamond),

sio2 (quartz)