1. when magnitude and direction of velocity do not change , acceleration of moving body is zero

2.This "negative" acceleration is actually called deceleration. It means that over time, you are slowing down instead of speeding up- your velocity is continously decreasing. To show this decrease, mathematically we put a negative sign to indicate this.

3.when the acceleration is in the direction of motion then there must be an positive acceleration.

Positive AccelerationWhen the velocity of an object increases, the acceleration is positive.In this case, the acceleration is in the direction of Velocity.

Negative Acceleration When the velocity of an object decreases with time,it has negative Acceleration.Negative acceleration is also called deceleration.Its directions is opposite to the direction of Velocity.

Zero Acceleration If the velocity of the object does not change with time,it has zero acceleration.

his answer is a and b both positive and negative because the when the velocity of an object increases the acceleration can produce and the acceleration is positive .and when the velocity of an object decrease within the time it has negative acceleration