1. Key idea: vector sum of forces

   If several forces act simultaneously on a particle, the acceleration is determined by the resultant force: $\vec F_{\text{net}} = \vec F_1 + \vec F_2 + \vec F_3$

2. Given condition

   The three forces are represented in magnitude and direction by the three sides of a triangle $ABC$.

   When vectors are represented by the three sides of a triangle taken in order, their vector sum is zero: $\vec{AB} + \vec{BC} + \vec{CA} = 0$

   Hence, $\vec F_1 + \vec F_2 + \vec F_3 = 0$

   Therefore, $\vec F_{\text{net}} = 0$

3. Effect on motion

   By Newton's second law, $\vec a = \frac{\vec F_{\text{net}}}{m} = 0$

   So the particle has zero acceleration.

4. Conclusion about velocity

   If acceleration is zero, the velocity does not change. Since the particle was already moving with velocity $\vec v$, it will continue to move with the same velocity: $\vec v\; \text{remains unchanged}$

5. Option check

   • A: less than $\vec v$ — incorrect, because no acceleration acts.
   • B: greater than $\vec v$ — incorrect.
   • C: $|v|$ in direction of the largest force $BC$ — incorrect, because resultant force is zero.
   • D: $\vec v$ remaining unchanged — correct.

Derived answer: Option D.