Final answer:
Option (b) y = x² is the function that has a positive value with a positive slope decreasing in magnitude at x = 3.
Step-by-step explanation:
The question is asking for the identification of a function that has a positive value and a positive slope that is decreasing at x = 3. Given the options provided, option (b) y = x² is the function that fits the description. This function indeed has a positive value for all positive x, and its slope, represented by its derivative 2x, is positive but decreases in magnitude as x increases from 3 to larger positive values.