The following shows a graph of the energy changes taking place as a chemical reaction progress. if an enzyme is added, which arrow's magnitude will change? will it increase or decrease?

The arrow representing the activation energy will decrease in magnitude.

This arrow will be beginning at the x-axis and going up to the maximum of the graph. The reason that its magnitude decreases is because enzymes provide  alternate pathways to reactions; the new pathway has a lower activation energy, so the reaction occurs faster when an enzyme is added. This is why enzymes are referred to as biological catalysts and are a vital part of an organism's body