Into which chamber of the heart will blood flow next after the vessel indicated by the red arrow

Right atrium

The atrium is the upper chamber in which blood gets into the heart. There are two of these in every human being and they both receive blood. the left chamber from the lungs, and the right from the venous course. In context of the question, the little cardiovascular vein discharges into the coronary sinus, which channels into the right atrium.