Analysis:

To determine the body's maximum acceleration upward, we need to first understand the forces acting on the body and then apply Newton's second law of motion.

Forces acting on the body:
1. Weight (W): The weight of the body is the force with which it is pulled downwards due to gravity. It can be calculated as the product of the mass (m) and the acceleration due to gravity (g). Hence, W = mg = 300N.

2. Normal Force (N): The normal force is the force exerted by a surface to support the weight of an object resting on it. In this case, when the body jumps upward, the normal force is greater than the weight, resulting in an increase in the scale reading. The normal force can be calculated as N = W + ma, where ma is the additional force applied to the body.

Applying Newton's second law of motion:

Newton's second law states that the net force acting on an object is equal to the product of its mass and acceleration. In equation form, F_net = ma.

Calculating the maximum acceleration:

In this scenario, the maximum acceleration occurs when the normal force is at its maximum value (400N). Therefore, we can rewrite the equation N = W + ma as 400N = 300N + m * a_max.

Simplifying the equation, we get a_max = (400N - 300N) / m = 100N / m.

To find the mass (m), we can use the equation W = mg. Rearranging the equation, m = W / g = 300N / 10m/s^2 = 30kg.

Substituting the value of mass into the equation for maximum acceleration, we get a_max = 100N / 30kg = 3.33 m/s^2.

Therefore, the body's maximum acceleration upward is 3.33 m/s^2.

Explanation:

When the body jumps upward, the normal force exerted by the scale increases to balance the additional force applied by the body. By analyzing the forces acting on the body and applying Newton's second law of motion, we can determine the maximum acceleration of the body. In this case, the maximum acceleration is found to be 3.33 m/s^2.