CONCEPT: The question tests on the basics of decision making and linear arrangement
SOLUTION:
Using Statement (1)- There are 8 more people in front of Jack than behind him and there are 4 people behind Jack.
=> If there are 4 people behind Jack there are 4+8=12 people ahead of Jack.=>17 people in the line.
(SUFFICIENT)
Using Statement (2) -There are three times as many people in front of Jack as behind him.
If there are x people behind Jack,x+8 ahead of Jack then,
3x = x +8
(SUFFICIENT) to solve.
Hence OPTION (D