In the context of the heat equation, particularly in thermodynamics and heat transfer, \( c \) typically represents the specific heat of the substance. The specific heat is a material property that defines the amount of heat required to change the temperature of a unit mass of the substance by one degree Celsius (or one Kelvin).
Thus, to clarify the options you provided:
- Heat required to raise the temperature: This is related to the specific heat but is not what \( c \) directly represents.
- Specific heat of the substance: This is the correct answer.
- Mass of the substance: This is typically represented by \( m \) in equations.
- Change in temperature: This is usually denoted by \( \Delta T \) or \( T_f - T_i \) (final temperature minus initial temperature).
Therefore, \( c \) in the heat equation represents the specific heat of the substance.