Which of the following factors enhances the myocardial contractility?

Myocardial contractility refers to the heart muscle's ability to contract effectively. Factors that enhance myocardial contractility improve the heart's ability to pump blood with greater force. Improved calcium availability is crucial for contractility, as calcium ions play a key role in the process of muscle contraction. When calcium levels are adequate or enhanced in cardiac muscle cells, it facilitates the interaction between actin and myosin filaments, leading to stronger and more efficient contractions.

Increased after-load can actually decrease contractility because it adds resistance the heart must work against, thereby potentially reducing the effectiveness of each contraction. Decreased preload reduces the volume of blood entering the heart, which can also lead to weakened contractions since the heart has less stretch and less volume to work with. A lower heart rate may allow for improved filling time per cycle, but it does not necessarily enhance the contractility itself; rather, it may facilitate better preload levels under certain conditions. Therefore, improved calcium availability directly supports and enhances the myocardial contractility, making it the correct choice.