Choosing response and predictors 
Information on the Girth (), Height () and Volume () for 31 cherry trees
yielded the following correlations : $r_{12}$ = 0.519,  $r_{23}$= 0.598 and  $r_{13}$= 0.967. If you were asked to setup a regression choosing a certain characteristic as the response
and the other two as predictors, which characteristic will you choose as the
response? Give your comments.

I think we choose Girth and Height as predictors for the response Volume because the correlation between Girth and Height is the least, so we're looking at how these individually affect the Volume (i.e. the factor with which their correlation is more than that between them).
Is my reasoning correct?
This question was asked in the final examinations of the course on Multivariate Data & Linear Models and it had 10 marks. What more should I write?

Predictors with high correlation are more linearly dependent. So they have a similar effect on the dependent variable. Thus doesn't make sense to use Height as the response. Choosing between Girth and Volume it seems more logical to use Volume. As you mentioned in the answer to @whuber, the volume might be the most difficult to measure in living trees.
