A

Step 1: Consider the acceleration of the trolley. The trolley moves horizontally with acceleration 'a'.
Step 2: According to the principle of relative motion, in the reference frame of the accelerating trolley, a pseudo force acts on the liquid in the opposite direction of the trolley's acceleration.
Step 3: This pseudo force causes the liquid to slant. The resultant of gravitational force acting downwards and the pseudo force acting horizontally represents the new effective 'weight' direction of the liquid.
Step 4: The surface of the liquid will be perpendicular to the resultant of these two forces.
Therefore, the surface configuration of the liquid will be tilted at an angle relative to the horizontal, higher at the back of the trolley and lower at the front.
So, the surface of the liquid in the vessel will be slanted upwards in the direction opposite to that of the acceleration.