The correct answer is $\boxed{\text{A}. 0.87}$.
The coefficient of correlation is a measure of the strength of the linear relationship between two variables. It is calculated by taking the covariance of the two variables and dividing it by the product of their standard deviations.
In this case, the estimated regression equations for $x$ and $y$ variables are $x = 0.85y$ and $y = 0.89x$. This means that the covariance of $x$ and $y$ is $0.85^2 = 0.7225$ and the standard deviations of $x$ and $y$ are $1$ and $1$, respectively. Therefore, the coefficient of correlation is $\frac{0.7225}{1 \cdot 1} = 0.7225 = 0.87$.
Option B is incorrect because the coefficient of correlation cannot be greater than $1$. Option C is incorrect because the coefficient of correlation cannot be less than $-1$. Option D is incorrect because the coefficient of correlation is not equal to $0.75$.