The correct option is
B tough
Option A - 'Simplified' is a comparative adjective, which means 'simpler'. As the given sentence already has 'very', which is used as a comparative degree of determiner, we can't use a comparative adjective. Thus option A is incorrect.
Option B - 'Correctness' is a noun, which means 'appropriateness of something.' For example, 'There was no evidence to support his correctness.' As 'very' is a determiner of degree, we can't use it with a noun. Thus option B is incorrect.
Option D - 'Tougher' is a comparative adjective, which means 'harder'.As the given sentence already has 'very', which is used as a comparative degree of determiner, we can't use a comparative adjective. Thus option D is incorrect.
Option C - 'Tough' is the positive degree of adjective, which means 'difficult/hard'. It can be used with a comparative determiner 'very'. Thus option C is the correct answer.
'The English paper was very tough.'