To treat the contraction of cardiac muscle, digitalis is used to:

Digitalis is a medication that is primarily used to improve the efficiency and contraction strength of the heart muscle. It increases the force of each contraction of the cardiac muscle, which is especially beneficial in conditions like heart failure or atrial fibrillation. This action is achieved through the inhibition of the sodium-potassium ATPase pump, which leads to an increase in intracellular sodium. The higher sodium concentration causes a subsequent increase in intracellular calcium, enhancing the myocardial contractility.

Thus, the primary purpose of digitalis is to increase the strength of cardiac contractions, making it easier for the heart to pump blood effectively. This is why the correct answer is that digitalis is used to increase the contraction of cardiac muscle.