Which is a characteristic of projectile motion? a) the forward motion negates air resistance. b) it follows a curved path. c) t has variable acceleration. d) it is unaffected by gravity.

The movement of a projectile is a movement in two dimensions (forming a curved path: a parabola shape) with constant acceleration.

A projectile is any body or object that is thrown or projected by means of some force and continues in motion by its own inertia. This means the only force that acts on it while in motion is the acceleration of gravity (in this case we are on Earth, so the gravity value is ).  

Where gravity influences the vertical movement of the projectile, while the horizontal movement of the projectile is the result of the tendency of any object to remain in motion at a constant speed (according to Newton's 1st law of motion sometimes called Law of Inertia).

The other options are incorrect because are false:

-The forward motion negates air resistance: There is always at least a small percent of air resistance, as long as that movement is done on Earth.  

-It has variable acceleration: In projectile motion acceleration is constant (gravity acceleration) .

-It is unaffected by gravity: The only force that acts on the projectile is due gravity.

diagram is missing, but i will try to explain your doubt.

answer:

amplitude is the maximum displacement or distance moved on a point of a vibrating body or wave measured from its equilibrium position.it is equal to half the length of vibration path.