Solution to question 2.2
The variable n is simply the number of subjects in this study group. It is a discrete metric variable as it is countable.
dmft is a discrete metric variable (you count the number of decayed missing and filled teeth. (Note that it is possible for the mean of a discrete variable to be any value, not just whole numbers.) Similarly Decayed/filled teeth and Extracted teeth are discrete metric variables.
% caries-free is a continuous metric variable. The value of this variable is infinitely variable (between the values of 0 and 100).
The four social class variables, as presented here look to be continuous metric variables (like % caries-free above) and, indeed this is how they were treated in the paper. The authors, however, made a mistake. These are not really four independent variables but four categories of one nominal variable, social class (or, possibly, four categories of one ordinal variable, if we choose to regard social class in that way.