Isostatic adjustments are the result of the buoyancy of earth's lithosphere as it floats on the below which is denser and plastic like.

Liquid Asthenosphere below which is denser and plastic like.

Explanation:

Word iso means equal and statos means standstill, it's the gravitational pull that is exerted by the earth from below.  

Such as the lithospheric adjustment this relationship is maintained through the earth's geological cycle.  

The concept given by Dutton in 1889. The example of mountain range block which on eroding get carried away if it's not heavier and will add to the geomaterial from below also known as crustal rebound theory.  

To maintain this balance, there must be a continuous supply of material from the upper plate's to bottom zones so that the isostatic balance is maintained.

Isostasy has been most active in glacial topography and gravitational pull, earth's rotation, and earthquakes are some of the important geoengineering processes.  

it sounds like either a or c

i believe this is