Julia has eaten one-quarter of the pizza. So three-quarters of the pizza remain.  Therefore the required ratio is $$\frac{3}{4}:\frac{1}{4}=3:1$$. 
Julia has eaten one-quarter of the pizza.  What is the ratio of the number of pizza that remain to the number Julia has eaten? 
$$1:3$$ 
$$3:1 $$ 
$$1:4 $$ 
$$4:1$$ 
$$4:3$$ 