How do you write a claim about your testable question?

In general, a claim is your best answer to your testable question that can be backed up by the evidence you have collected in your experiment. You should also be able to justify your claim with reasoning based on any science concepts you can apply to the situation. This follows the claim-evidence-reasoning model of creating a scientific argument. 

For example, an experiment I enjoy using at the beginning of the year with my students involves termites. I begin by demonstrating how a termite will follow a line I draw on a paper with a ball point pen. From that point, students generate all different kinds of questions about why the termite follows the line. Some say it's by sight, some say it's by scent, and others say it's by touch. Each group of students has their own testable question that they've developed and they decide how they will test it and collect data.

After the experiments are conducted and data is collected, students then make a claim. For example, one group may claim that the termites follow the pen line by smell because when they tried other brands of ball point pens, the termites wouldn't follow the line. They have evidence to back this up because they collected data during the lab, and they have some sort of reasoning to connect everything together based on their background research of animal behavior and different brands of pens.

I hope this helps!