What is the common ratio of the geometric sequence 1, 4, 16, 64,

A geometric sequence refers to a sequence where to get from one term to another you need to multiply by the same number each time. This number is called common ratio or factor. 

On the case are given the numbers 1,4,16,64 as part of the sequence.

From 1 to 4 multiply by 4
From 4 to 16 multiply by 4
From 16 to 64 multiply by 4
 
After the previous reasoning, you can conclude that the common factor or ratio of the geometric sequence is 4.

For Example;
1) 
2) 

For this sequence the common ratio = 4