Geosyncline is defined as:

Answer: [A] a linear trough of subsidence of the Earth'€™s crust where sediments collect

First option is the correct answer
Geosyncline is subsidence of some part of the landmass. The Geosyncline is then filled up by the deposition of sediments brought by agents like rivers and glaciers. The mountain building process starts with geosyncline.

This question is a part of GKToday's Integrated IAS General Studies Module