A linear function is any function that graphs to a straight line.

The function in option F is $\"\"$ .

$\"\"$ is a quadratic function. it represents a upward parabola in a coordinate plane.

The function in option G is $\"\"$ .

$\"\"$ is a constant function. it represents a horizontal line in coordinate plane.

The functions in remaining options H and J $\"\"$ and $\"\"$ are square root function.

These are also represent a nonlinear graphs in a coordinate plane.

Therefore, in the four options the constant function $\"\"$ is a linear function.

Option G is the correct choice.