For each coefficient choose whether it is positive or negative.Choose the coefficient with the least value.Choose the coefficient closest to zero.

From the given graph
a)
If the coefficient is positive, then the graph is upward
If the coefficient is negative, then the graph is downward
In the first 2 figures, the graphs are upward, then
A and B are positive
In the last 2 figures, the graphs are downward, then
C and D are negative
b)
The least coefficient is the coefficient of the graph which nearest to the x-axis
Since graph C is the nearest graph to the x-axis, but we have negative values, then
The least coefficient is the coefficient of the downward graph and farthest from the x-axis, then
D is the least coefficient
c)
The closest coefficient to zero is the graph C