According to the ideal gas law, \( PV = nRT \), where \( P \) is pressure, \( V \) is volume, \( n \) is the number of moles of gas, \( R \) is the ideal gas constant, and \( T \) is the temperature in Kelvin.
If the volume \( V \) is held constant and the temperature \( T \) increases, then the equation can be rearranged to show that pressure \( P \) is directly proportional to temperature:
\[ P = \frac{nRT}{V} \]
Since \( n \), \( R \), and \( V \) are constant, an increase in \( T \) will result in an increase in \( P \).
Therefore, the correct answer is:
D. It would increase.