Go through the options one by one and eliminate the vectors.
Force needs a direction to make sense, a push has to push somewhere, so force is a vector.
Velocity is speed plus a direction of travel, so it is a vector.
Acceleration is the rate at which velocity changes, and this change also has a direction, so acceleration is a vector too.
That leaves pressure. Pressure is force divided by area, and at any point in a fluid it pushes equally outward in all directions, so a single magnitude fully describes it with no attached direction.
By elimination and by definition, pressure is the scalar quantity, so option B is correct.