An object is moving in the absence of a net force. which of the following best describes the object's motion? - ?

You can solve this by using Newton's First Law or Newton's Second Law.

1) Newton's First Law or Inertia Law states that in the abscense of a net force acting over an object, this will not chage its state of movement, i.e it will  remain at rest (if it is no moving) or with uniform movement (if the object is moving).

2) Newton's Second Law: Net force = mass * acceleration => acceleration = net force / mass = 0 / mass = 0.

Zero accelerations means rest or uniform movement.

First Law is implicit in Second Law.

There are two cases in which no net force is being applied on an object:
1) The object is stationary
2) The object is not accelerating; that is, the magnitude or direction of its velocity is not changing.
Because this question requires the object to be in motion, the velocity of this object must be constant but non-zero.
Therefore, the correct option would be the one where the velocity is constant and in a straight line. (circular motion means the velocity's direction is constantly changing, thus the velocity is not constant)

Gas will most likely  fill the container