Questions: The active site of an enzyme 1. never changes shape. 2. forms no chemical bonds with substrates. 3. determines, by its structure, the specificity of the enzyme. 4. looks like a lump projecting from the surface of the enzyme. 5. changes the ΔG of the reaction.

The answer is the third one (3): determines, by its structure, the specificity of the enzyme.

Explanation for each option:

1. Never changes shape: This is incorrect. The active site of an enzyme can change shape slightly when the substrate binds, a concept known as "induced fit."

2. Forms no chemical bonds with substrates: This is incorrect. The active site often forms temporary chemical bonds with the substrate to facilitate the reaction.

3. Determines, by its structure, the specificity of the enzyme: This is correct. The structure of the active site is specific to the substrate it binds, determining the enzyme's specificity.

4. Looks like a lump projecting from the surface of the enzyme: This is incorrect. The active site is typically a pocket or groove on the enzyme's surface, not a lump.

5. Changes the ΔG of the reaction: This is incorrect. Enzymes do not change the free energy change (ΔG) of a reaction; they only lower the activation energy, making the reaction proceed faster.

In summary, the active site's structure is crucial for the enzyme's specificity, making option 3 the correct choice.