What was Mohorovičić's observation?

Seismic waves travelled at constant speeds.

Seismic waves penetrated all Earth's layers.

Seismic waves could not go through the brittle crust.

Seismic waves travelled at varying speeds.

Seismic waves traveled at varying speeds.

Explanation:

The Moho Discontinuity was discovered by one Croatian seismologist, Andrija Mohorovicic. He realized that the speed of a seismic wave depends on the density of the material the wave is moving through. Mohorovicic interpreted the acceleration of seismic waves observed within the Earth's crust as a change in composition of matter within the Earth.

Option (4)

Explanation:

Andrija Mohorovicic was a famous seismologist from Croatia. He observed that the seismic waves namely P and S waves travel through the interior of the earth. The speeds at which these waves travel is different in the different layers of the earth. The P wave was observed to be travelling at a faster rate than the S wave and it can travel in both solids as well as liquid i. e it can travel through the crust, mantle and core region. Whereas the S wave can travel only in crust and mantle. This variation in speed occurs due to the increasing density towards the inner core of the earth.

Thus the correct answer is Option (4).