Scientists make use of two types of experiments while investigating on hypothesis. Two types of experiments include controlled experiments and data collection (field study).
In controlled experiment, only one variable is kept constant at a time. If all the variables are changed, then it will not be possible to study the effects of desired variable. Controlled experiments cannot be performed on the living organisms only in some cases. Scientist cannot always use controlled experiments. If a biologist wishes to study the interaction between organisms in its natural environment, he cannot use controlled laboratory experiments. Instead he has to opt for field study method.
Hence, the correct answer is option (b) only one variable is tested at a time.