Answer: The rate at which he is willing to substitute one good for the other
Explanation: Indifference curve shows the combination of two goods that give the consumer the same level of satisfaction. the slope of this indifference curve shows how much the consumer is willing to substitute one good for the other in order to keep utility constant.
Slope of Indifference curve for soda and chips shows how much soda Timothy is willing to substitute to get 1 additional unit of chips.
So, the correct option is the rate at which he is willing to substitute one good for the other.