Final answer:
The effectiveness of digoxin (Lanoxin) treatment is indicated by improved cardiac contractility, which can in turn reduce exercise intolerance due to better blood circulation.
Step-by-step explanation:
The question focuses on the effectiveness of digoxin (Lanoxin) treatment. Digoxin is known to increase the force of cardiac contraction and is therefore considered a positive inotrope. Thus, an indication that digoxin treatment is effective would be improved cardiac contractility (option c). This improvement can lead to a stronger and more efficient heartbeat, thereby improving symptoms such as reduced exercise intolerance through better blood circulation and oxygen delivery to tissues.
It's important to note that while digoxin improves cardiac contractility, other drugs such as diuretics might be used to treat hypertension or heart failure through different mechanisms, such as decreasing the reabsorption of Na+ and Cl- in the loop of Henle.