Answer:
1, 2, 4
Step-by-step explanation:
Consider all options:
1. 
Express y:

This is the equation in the form

so this is a linear function.
2. 
can be rewritten as

This is the equation in the form

so this is a linear function.
3. 
This function is equivalent to the function

This is quadratic, not linear function.
4. 
This is the equation in the form

so this is a linear function.
5. 
This is not linear function, because x is in the denominator.
6. 
This is not linear function because there is a squer root of x.