How do I find the least optimised query?
My relational algebra is pretty rusty but maybe I can put you on the right track. Start with a simpler problem: What if you were looking for book number "ZZZZ81-203-1257-0" which had no matches in the tables? Which query would be the least optimized then?

Let's randomly consider the third relational algebra expression as an example. You are doing a cartesian join between the member and lending tables so you'll end up with 1000 x 400 = 400000 rows as an intermediate result set. You then check the filter condition on each of the 400000 rows and don't keep any of them. Does that seem efficient or inefficient to you? How does it to compare to the others? Can you draw any comparisons between the reasoning that you used for this case compared to the problem that you're trying to answer?