The percentage of the variance in Y explained by X is given by the square of the correlation coefficient (r) between X and Y.
In this case, the value of r between X and Y is 0.90. To find the percentage of the variance in Y explained by X:
Percentage explained variance = r^2 * 100
Percentage explained variance = (0.90)^2 * 100
Percentage explained variance = 0.81 * 100
Percentage explained variance = 81%
So, the correct answer is 81%.