Which of the following temperatures is the coldest? a) 5 degrees b)0 degrees c) -15 degrees d) -7 degrees

Well, it's most likely choice-C, but ONLY if the choices are all measured on the same temperature scale.  

For example . . .

--> 5 degrees Fahrenheit is colder than 0 degrees Celsius

--> 0 degrees Fahrenheit and 5 degrees Fahrenheit are both colder than -7 degrees Celsius, AND they're both colder than -15 degrees Celsius

i guess that visual aids present persuasive examples to what you are presenting, some factors would be theme (does it fit with the color, information, what you are presenting), size (can you see it, is it a big enough span of data/information), and overall look of the visual aid.

hope this : )