Thank you for your enquiry. I've had a quick look at the question in the book and your thread, and I can see where the confusion may lie.
The question asks for an estimate of the 12th highest salinity level. The highest salinity level would clearly be 15.95 from the cumulative frequency graph. Now you must work backwards from this to find the 12th highest level which lies at approx 11.4. This would indeed be the 18th from the lowest salinity level (as you have indicated in your 3rd answer possibility).
It seems that this is mainly an interpretation issue - you clearly understand the main mathematical principles in this question.
Enjoy working through the rest of the book.