What happens during a controlled experiment

a. no observation are made

b. many variables are changed

c. a factor called prediction is changed

d. the results of changing independent variable are observed

what is the correct answer
for this question

The correct answer is D.

In a controlled experiment you have two variables: independent and dependent.
The independent variable is what you're changing, and the dependent variable is the result you get. The dependent variable "depends" on what you make the independent variable.

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.