Which of the following is the primary treatment for heart failure to improve contraction strength?

The primary treatment for heart failure aimed at improving contraction strength is digitalis. Digitalis, also known as digoxin, is a cardiac glycoside that increases the force of myocardial contraction (positive inotropic effect). This mechanism is particularly beneficial in heart failure patients, as it enhances the heart's ability to pump blood more effectively, leading to improved cardiac output and alleviation of symptoms associated with heart failure.

In contrast, the other options serve different roles in the management of heart failure. Diuretics primarily help to reduce fluid overload and alleviate symptoms of congestion by promoting the excretion of sodium and water. Beta-blockers may decrease heart rate and reduce myocardial oxygen demand, providing a protective effect on the heart, but they do not directly improve contractility. ACE inhibitors are important for managing heart failure by providing afterload reduction and improving hemodynamics, but they also do not directly enhance the strength of heart muscle contractions. Therefore, digitalis remains the cornerstone for enhancing contraction strength in the context of heart failure.