Hello there! The answer is a) a poorly designed experiment.
You want to have only ONE variable in an experiment so you can test how that one thing interacts with or changes other things. For example, if you are testing to see what type of material seeds grow best in, you choose different growing materials (like sand, mud, gravel etc.) but you want to keep the type of plant seed, container, how much water each plant gets - all the same since your variable is the type of growing material.
Hope this helped! :)
