The linear equation among the provided options is:
A) \( y = x \)
A linear equation is one that can be expressed in the standard form \( y = mx + b \), where \( m \) and \( b \) are constants. \( y = x \) fits this definition, with \( m = 1 \) and \( b = 0 \).
The other options represent non-linear equations:
- B) \( y = |x| \) (absolute value function)
- C) \( y = x^2 \) (quadratic function)
- D) \( y = \sqrt{x} \) (square root function)