Which theory asserts that each individual has an ideal body weight that is resistant to change?

Set point theory.

Explanation:

Set point is defined as the weight range in which your body is programmed to function optimally. Set point theory holds that one's body will fight to maintain that weight range that is, our body is biologically and genetically determined to weigh within a certain weight range. The set point model relies on the concept of a genetic preset weight range that’s controlled by biological signals. The body has a regulatory system that keeps you at a steady-state level, or set point.

Your hypothalamus, which is in your brain, gets signals from fat cells. Hormones like leptin, which regulates hunger, and insulin are triggered at certain times. Your metabolism also constantly adjusts up or down based on a variety of signals. The set point theory suggests that your weight may go up or down temporarily but will ultimately return to its normal set range. The signaling system helps maintain weight.