Which describes the action of a ligand's competitive antagonist?

After a quick search, I was able to find a source with your question and its multiple choice answers

Next time, you should include them, it is helpful! 

"Which describes the action of a ligand's competitive antagonist?"

-A.) It decreases the amount of ligand required to activate the protein.

-B.) It blocks the binding of the ligand to its binding site and prevents activation of the protein.

-C.) It binds to a different binding site on the protein, altering the protein shape so that a ligand cannot bind.

D.) When present, it increases the binding of a ligand to its receptor.

Your answer would be "B". 

The answer would be true