Final answer:
Decreasing the volume of the reaction vessel is likely to increase the rate of reaction because it increases pressure and the frequency of collisions between reacting particles.
Step-by-step explanation:
The question asks which change is likely to increase the rate of reaction in a reaction mixture. Decreasing the volume of a reaction vessel is likely to increase the rate of reaction as this leads to an increase in pressure due to a reduction in available space. According to the collision theory, a higher concentration of particles or a smaller volume can both result in an increased rate of reaction.
This is because more particles, or fewer spaces, can increase the probability of collisions between reacting particles, which is a key factor in the speed of chemical reactions. Therefore, the answer is that decreasing the volume of the reaction vessel can increase the reaction rate.