In which of the following reaction will a change in pressure most likely to affect the rate of reaction

A change in pressure primarily affects the rate of reactions that involve gaseous reactants. Increasing the pressure increases the concentration of the gaseous reactants, which leads to a higher frequency of collisions between reactant molecules and thus a faster reaction rate. The greater the number of moles of gaseous reactants, the more significantly the rate will be affected by a change in pressure.

Let's examine the gaseous reactants in each option:

• A. $\text{C}_{(\text{s})} + \text{O}_{2(\text{g})} \rightarrow \text{CO}_{2(\text{g})}$ There is 1 mole of gaseous reactant ($\text{O}_2$).
• B. $\text{N}_{2(\text{g})} + 3\text{H}_{2(\text{g})} \rightarrow 2\text{NH}_{3(\text{g})}$ There are $1 + 3 = 4$ moles of gaseous reactants ($\text{N}_2$ and $\text{H}_2$).
• C. $\text{Mg}_{(\text{s})} + \text{O}_{2(\text{g})} \rightarrow \text{MgO}_{(\text{s})}$ There is 1 mole of gaseous reactant ($\text{O}_2$).
• D. $\text{Na}_{(\text{s})} + \text{Cl}_{2(\text{g})} \rightarrow 2\text{NaCl}_{(\text{s})}$ (Note: This equation is unbalanced, it should be $2\text{Na}_{(\text{s})} + \text{Cl}_{2(\text{g})} \rightarrow 2\text{NaCl}_{(\text{s})}$, but the number of moles of gaseous reactant remains the same for the purpose of this question.) There is 1 mole of gaseous reactant ($\text{Cl}_2$).

Reaction B involves the largest number of moles of gaseous reactants (4 moles). If the pressure is increased, the concentrations of both $\text{N}_2$ and $\text{H}_2$ will increase. For example, if the rate law for this reaction is $\text{Rate} = k[\text{N}_2][\text{H}_2]^3$ (assuming an elementary step or a rate-determining step with these stoichiometries), doubling the pressure would double both $[\text{N}_2]$ and $[\text{H}_2]$, leading to a rate increase by a factor of $2 \times (2)^3 = 2 \times 8 = 16$. In contrast, for reactions A, C, and D, where there is only 1 mole of gaseous reactant, doubling the pressure would typically increase the rate by a factor of 2 (assuming first-order dependence on that gas).

Therefore, the rate of reaction B will be most significantly affected by a change in pressure.

The final answer is $\boxed{\text{B}}$.