A simple pendulum is attached to a block which slides without friction down an inclined plane (ABC) having an angle of inclination $\alpha$ as shown
while the block is sliding down the pendulum oscillates in such a way that at its mean position the direction of the string is:

• A

  at angle $\alpha$ to the perpendicular to the inclined plane $A C$.
• B

  parallel to the inclined plane AC.
• C

  vertically downwards.
• Correct answer
  D

  perpendicular to the inclined plane AC.

Hence, the correct answer is option: 'D'.