S+T = 0

only way this could be possible, when lsl = ltl, but either s or t has to be negative and other one has to be positive

Statement I

ls-0l = l0-tl
lsl = ltl

Sufficient.

Statement II

0 is between s and t,

s could be -2 and t could be 1

or

s could be -1 and t could be 1

In both cases 0 is between s and t.

Insufficient.

Hence A.

Hope this helps.

Actually in this question I guess it's easy to miss that s and t are 2 different numbers so s cannot be equal to t and that's why 'A' is the answer.