Opposite charges attract; like charges repel :)
Answer: Trough
Explanation: The point labeled C in the wave diagram above is the TROUGH of the wave motion. The trough of a wave motion identifies or signifies the point of least or minimum Displacement by measuring the downward Displacement of the wave. The point A is the CREST which is the opposite of the trough, signifying the point of maximum or upward Displacement of the wave cycle.
Point B is the wave amplitude which signifies the maximum extent of vibration from the equilibrium position of a wave. The point labeled D refers to the wavength of the wave motion which is the distance between successive crest or troughs of a wave motion.
I do believe all of these but core elements can be determined by spectroscopy which includes the use of electromagnetic radiation. Both the surface and core temperature can be measured using light. Surface elements can be found because the absorption lines of different elements in the spectra of the star, but I haven't heard anything about using spectral analysis for core elements.
(a) 
The frequency of a wave is given by:
where
v is the wave's speed
is the wavelength
For the red laser light in this problem, we have
(speed of light)
Substituting,
(b) 427.6 nm
The wavelength of the wave in the glass is given by
where
is the original wavelength of the wave in air
n = 1.48 is the refractive index of glass
Substituting into the formula,
(c) 
The speed of the wave in the glass is given by
where
is the original speed of the wave in air
n = 1.48 is the refractive index of glass
Substituting into the formula,
The legend is that he discovered gravity when an apple feel on his head. I don’t know what the true story is, but that’s what I’ve heard so maybe A??
Although, I’m pretty sure it could also be C
So... between A and C, however, I don’t want you to get it wrong so I would recommend getting another opinion
Hope this helps!
