A ball is thrown vertically upwards. Which of the following graph/graphs represent velocity-time graph of the ball during its flight (air resistance is neglected)    

For a ball thrown vertically upward with air resistance neglected, the acceleration is constant and equal to $-g$ throughout the flight. Therefore the velocity changes linearly with time. So the $v$-$t$ graph must be a straight line with constant negative slope. Initially the velocity is positive, then it becomes $0$ at the highest point, and after that it becomes negative while the ball comes down. Hence the correct graph is the one straight line sloping downward and crossing the time axis once. Among the given figures, this matches option $D$.