Which answer is the best estimate of the correlation coefficient for the variables in the scatterplot?

-0.8 would be the best estimate.

The data seem to have a negative correlation, since they are decreasing; this means our r-value will be negative.

Our two negative choices are -0.25 and -0.8.  -0.8 would be a fairly strong correlation, whereas -0.25 would be a weak correlation.  The data do seem to be fairly strongly related, so we will use -0.8.