Hello!
The graph of f(x) = sin(x) is an odd function, while the graph of f(x) = cos(x) is an even graph. As shown on the graph above, this graph is even because it is symmetric with respect to the y-axis.
Also, the range of this function is -3 ≤ y ≤ 3, this tells us our amplitude. which is 3. Amplitude is the maximum displacement on the graph of a function.
Therefore, the equation of this graph is choice D, f(x) = 2cos x.
