Final answer:
All of the above options: the size of the skateboard wheels, the height of the ramp, and the weights on the skateboard are acceptable variables for manipulation in an experiment investigating skateboard speed down a ramp. The correct answer is d. All of the above are acceptable variables.
Step-by-step explanation:
In an experiment investigating how fast a skateboard rolls down a ramp, various factors can be manipulated to test their effects on the skateboard's speed.
These factors are known as variables. In this context, the size of the skateboard wheels, the height of the ramp, and the weights on the skateboard could all be variables.
In a scientific experiment, there are generally two kinds of variables: independent variables, which are the ones that are manipulated, and dependent variables, which are the outcomes that are measured in response to the changes made to the independent variables.
Often, experiments will also include control variables, which are kept constant to allow a clear understanding of how the independent variable affects the dependent variable.
So, to answer the question, all of the above options, a) the size of the skateboard wheels, b) the height of the ramp, and c) the weights on the skateboard, are acceptable variables that could be manipulated in an experiment to see their effect on how fast a skateboard rolls down a ramp.