The kinetic molecular theory assumes that the particles of an ideal gas
(1) are in random, constant, straight-line motion
(2) are arranged in a regular geometric pattern
(3) have strong attractive forces between them
(4) have collisions that result in the system losing energy

Option no.1 are in random, constant, straight-line motion

Following assumptions:
 a. number of molecular size is very large, but their separation is very large compared to their molecular size.
 b. randomly distribution of molecules in which their speed does not change 
c. molecules undergo elastic collision with other molecules but exert no force
d. molecules followed Newton's laws of motion