A

Explanation:
Internal forces are forces that the particles of a system exert on each other. By Newton's Third Law, these internal forces are equal and opposite. Consequently, when considering the entire system as a whole, the effects of internal forces will cancel out, and thus they do not change the total momentum of the system.

Step 1: Understanding momentum: The momentum of a system of particles is the vector sum of the momenta of each particle, given by:
$$ p = \sum_{i=1}^{n} m_i \vec{v}_i $$
where $m_i$ is the mass and $\vec{v}_i$ is the velocity of the $i^{th}$ particle.

Step 2: Analyze internal forces: Internal forces, by definition, act between the particles of the system. For instance, in a system of two particles, if particle A exerts a force on particle B, simultaneously, particle B exerts an equal and opposite force on particle A.

Step 3: Use Newton's Third Law: This means that the total force due to internal interactions sums to zero when accounting across the entire system: $$ \vec{F}_{\text{internal}} = \vec{F}_{A \to B} + \vec{F}_{B \to A} = 0 $$
Step 4: Applying the principle of momentum conservation: According to the principle of conservation of momentum, if no external forces are acting on the system, the total momentum remains constant despite any internal forces that may act.

Therefore, while internal forces can change the momentum of individual particles within the system, they do not change the total momentum of the entire system of particles.