Final answer:
The function that correctly expresses the height y as a function of time x since the bird left the branch is y=0.6sin(12πx)+35.
Step-by-step explanation:
The correct function that expresses y as a function of x is a. y=0.6sin(12πx)+35. This equation gives the height y in meters of a bird as a function of time x in seconds since the bird left the branch. The sine function indicates a periodic motion, 12π indicates the frequency of the oscillation, 0.6 represents the amplitude, and the constant 35 represents the average height above the branch. Therefore, a is the correct option and already expresses y as a function of x.