This is tricky, but the correct answer is A.
Choice-D is also true. We can't COMPLETELY determine velocity
from the graph, because an essential part of velocity is the DIRECTION
of motion, and the graph doesn't tell us anything about the direction.
But even though we can't COMPLETELY determine velocity from
the graph, we DO know that velocity is changing, because the speed
is definitely changing, and speed is the other big part of velocity.
The graph shows clearly that speed is changing. That right there
tells us that velocity is also changing, and we can pick choice-A.