Cells with similar structures and functions work together to form
a. tissues.
b. systems.
c. organisms.
d. organs.

Tissues. 
When large groups of tissues form up, they create tissues which the body will use to form organs.
Here is the cell levels in order for future reference:
Cells - Tissues - Organs - Organ Systems - Organisms

The basic unit of a living organism is a cell. Cells with the same function and structure comprises to makes up a tissue. A group of tissues combined with similar structure and forms makes up an organ. A group of organs with relative function makes up a system. Lastly, a group of system with relative functions make up an organism. To simplify:
Cell--> Tissue--> Organ--> System --> Living Organism.

The scientific method is a series of steps followed by scientific investigators to answer specific questions about the natural world. it involves making observations, formulating a hypothesis, and conducting scientific experiments.the steps of the scientific method are •observation-the first step of the scientific method involves making an observation about something that interests you•question-once you've made your observation, you must formulate a question about what you have observed. •hypothesis-once you've made your observation, you must formulate a question about what you have observed. •experiment-once you've developed a hypothesis, you must design and conduct an experiment that will test it.•results-the results are where you report what happened in the experiment. •conclusion-the final step of the scientific method is developing a conclusion.i hope that this you : )

They create genetic variations