For checking any functions to be invertible 
we use horizontal line test
Horizontal line test (HLT):
If a horizontal line intersects a function's graph more than once, then the function is not one-to-one
And For any function to be invertiable , it must be one to one 
so, we will verify each graph 
Graph-1:
we can see that horizontal line intersects on the graph at two points 
so, this is not one to one 
so, this is not invertiable
Graph-2:
we can see that horizontal line intersects on the graph at only one point 
so, this is one to one 
so, this is invertiable
Graph-3:
we can see that horizontal line intersects on the graph at two points 
so, this is not one to one 
so, this is not invertiable
Graph-4:
we can see that horizontal line intersects on the graph at only one point 
so, this is one to one 
so, this is invertiable