j and l are two rational numbers. The sum of 2 rational numbers is always rational. For example:
2 is rational, and 3 is rational. 2 + 3 = 5 and 5 is rational. So rational + rational = rational. So the first statement is incorrect, while the fourth is correct.
k and m are irrational numbers. The sum of 2 irrational numbers isn't definitive. It could be rational (for example (2+√2) is irrational and (2-√2) is irrational, but (2+√2) + (2-√2) = 4, and 4 is rational) and it can be irrational (√2 is irrational and √3 is irrational. √2 + √3 = 3.14 with an endless number of digits, which is irrational). So the second statement and third statement is incorrect.
All in All, the fourth statement is always true: The sum of 2 rational numbers is always rational.
Additional information:
The sum of 2 rational numbers can never be irrational.
The sum of 2 irrational numbers is sometimes rational, sometimes irrational.
Hope this Helps! :D