Hi kanika123,

DS questions test many different skills, one of which is your ability to make deductions when you only have a limited amount of information.

In this question, we're told that there are 4 variables (w, x, y and z) and we're asked which is biggest.

Fact 1 tells us y < z < x BUT we don't know anything about the value of w.
This fact ultimately means that either x is biggest (it's bigger than y and z and MIGHT be bigger than w) or w is biggest
Fact 1 is INSUFFICIENT

Fact 2 tells us that x = 25% of the total We don't know anything about the other 3 values (w, y and z), so there's no way to answer the question.
This fact ultimately means that x can't be biggest (it could be a 4-way tie, if they're each 25%, but if the others were all less than 25% then the math wouldn't add up to 100%, so x can't be biggest).
Fact 2 is INSUFFICIENT

Combining Facts, we know that x = 25%, y and z are both less than 25%, so the w has to account for everything else...

eg. 23% < 24% < 25% and the w would be everything else (in this case = 28%)
We have enough info to deduce that the w MUST be biggest

Final Answer: C

GMAT assassins aren't born, they're made,
Rich