Which options (ABC) has grammar errors and if no errors (select D)

Section 1
A. We discussed about the problem so thoroughly
B. on the eve of the examination
C. that I found it very easy to work it out.
D. No error

Section 2
A. An Indian ship
B. laden with merchandise
C. got drowned in the Pacific Ocean.
D. No error

Section 3
A. I could not put up in a hotel
B. because the boarding and lodging charges
C. were exorbitant.
D. No error

Section 4
A. The Indian radio
B. which was previously controlled by the British rulers
C. is free now from the narrow vested interests.
D. No error