The difference between spoken and written language are that spoken language uses shorter, simpler, and more familiar words than does written language
a) true
b) false

The answer would be a) true

When you write something formal, like an essay, you use written language with a formal register. When you speak, the language you use is much more colloquial, and so uses a simper version of the language, with shorter words. Colloquial language also tends to disregard the grammar of formal language. 

Ex.
Written: What do you think the weather will be like out tomorrow? 
              The weather will most likely be sunny with a few clouds out.
Spoken: What's the weather like tomorrow?
               Probably sunny with clouds or something.

As you can see with the examples, written language is much more formal, following grammar rules. Spoken language is very informal.

Hope that helps!

Bis the best answer.