The correct option is D I'd known
The given sentence is a type 3 conditional, it begins with 'if'. 'If' means 'on condition that.
A type 3 conditional presents an unreal past condition 'if I (know) sooner)' with its probable result 'would have tried' in the past. The if-clause is in the past perfect tense and the main clause is a perfect conditional 'I would have tried to help'. (We know the sentence is a type 3 because of the tense in the main clause.)
Option D: 'I'd known' is the contraction of 'I had known'. It'sin past perfect tense and is used in type 3 conditionals. It forms a complete meaningful sentence and hence D is correct.
Option A: 'I know' is in simple present tense.
Option B: 'I'll know' presents the future.
Option C: 'I knew' is in simple past.
These tenses are not used in type 3 conditionals, hence options A, B and C are wrong.