Given that X -> 1.1X and in the same time Y -> 0.9Y,
We need to find 0.9Y is what percent of X. or in other words relations between X and Y.

Statement 1: 0.1X = Y - Sufficient

Statement 2: 0.1X = 10/11 * 0.1 Y - Sufficient

As both statement independently provide us relation between X and Y.
Ans should be D.

I have an issue trying to interpret the stems.

(1) The increased price per share of stock X...
-and-
(2) The increase in the price per share of Stock X...

Do these statements mean the same thing?

To me, (1) means 1.1X, the increased price per share of stock X, and therefor 1.1X=Y

and (2) means .1X, the increase in the price per share of stock X, and therefor .1X=(10/11)(.1Y)

Although this doesn't alter the solution, I would like to know if I'm reading the stems correctly.

yeah your interpretation is right.

OA: D

(D)
We need to find 0.9y/x*100

Statement (1) 1.1x-x = 10/11 (y-0.9y) (POSSIBLE)

Statement (2) x(1.1 - 1) = 10/11 y (1-0.9) (POSSIBLE)