How is the kinetic energy of a moving object affected if the net work done on it is positive?

A particle is executing simple harmonic motion. Which one of the following statements about the acceleration of the oscillating particle is true?

Which one of the following four particles, whose displacement x and acceleration a_x are related as follows, is executing simple harmonic motion?

If we plot a graph between volume V and inverse of pressure p (i.e. $${1 \over p}$$) for an ideal gas at constant temperature T, the curve so obtained is