Even though the individual molecules in a system may have non-zero momentum, the total momentum of a system at rest is equal to?

Zero

Explanation:

Momentum is the product of mass and velocity.

momentum = mv

For a system at rest, the velocity = 0

momentum = m*0

= 0

Even though the individual molecules in a system have some momentum, the system has no momentum because the system is at rest and is left standing.