The correct option is A alter
Option A: 'Alter' means 'change in composition'. The word applies here: he wanted to change the terms of the agreement.
Hence option A is correct.
Option B: 'Adjust' means 'to move or regulate slightly in order to achieve the desired result'. This word isn't very suitable, compared to 'alter'. 'Adjust' means 'to change in order to reach a desired place' whereas 'alter' simply means 'to change'. The latter is more suitable for an agreement.
Hence B is incorrect.
Option C: 'Redo' means 'to do something again or differently'.
Option D: 'Exchange' is an act of giving one thing in return for another.
These words do not agree with the given sentence.
Hence options C and D are incorrect.