A50-kilogram student, standing on the earth, attracts the earth with a force closest to

The gravitation acceleration on the earth surface is equivalent to 10 N per kg.
Therefore a student with a weight of 50 kg exerts a force equivalent to;
  the mass of the student multiplied by the gravitation acceleration
 50 kg × 10 N/kg = 500 N
thus the force/weight exerted on the earth surface is 500 N

Wavelength divides in half and speed stays the same.