A block of mass \( m \) is attached to an ideal horizontal spring with spring constant \( k \). The block is placed on a frictionless surface and the spring is attached to a rigid wall. At time \( t = 0 \), the block is at the equilibrium position \( x = 0 \) and is given an initial velocity \( v_0 \) in the positive x-direction, causing it to undergo simple harmonic motion with an angular frequency \( \omega \). Which of the following expressions represents the acceleration \( a(t) \) of the block as a function of time \( t \) for \( t > 0 \)?