The answer will depend on the variables plotted on the graph!
acceleration
A straight line.
change in velocity
Magnitude of acceleration (but conveys no informationregarding acceleration's direction).
a) A circle is not the graph of a function. b) A circle is not linear.
acceleration
Indirectly, yes. If the graph is a straight line there is no acceleration, if the graph is not linear there is acceleration.
The slope of a speed-time linear graph represents acceleration. It indicates how quickly the speed of an object is changing over time.
A straight line.
One of the most common ways to represent linear equations is to use constants. You can also represent linear equations by drawing a graph.
The slope of a velocity-time graph represents acceleration.
Not necessarily. The equation of a projectile, moving under constant acceleration (due to gravity) is a parabola - a non-linear equation.
change in velocity
To find acceleration from a speed-time graph, you need to calculate the slope of the speed-time graph. The slope at any point on the speed-time graph represents the acceleration at that specific time. If the speed-time graph is linear, then the acceleration will be constant. If the speed-time graph is curved, you can find the acceleration by calculating the slope of the tangent line at a specific point.
The slope of a velocity-time graph represents acceleration.
Magnitude of acceleration (but conveys no informationregarding acceleration's direction).
Magnitude of acceleration (but conveys no informationregarding acceleration's direction).