I think you should drop the class if it's going to bring down your GPA. On the other hand, you should do fine in nursing school if your sciences are okay.
I think your instructor does have a good point though. The "thinking" that is needed in science courses is basically learning the techniques and apply them to solve problems. We're usually given x and y, and ask to find z. Critical thinking is a bit different. You assess the evidence, assumptions, and implications, then build an argument or offer alternatives. Answers aren't absolute like those in sciences/math, which led to the misconception that it's subjective. I guess that science would require this type of thinking when you reach a higher level and in research. I know that business requires this kind of skill when you're performing research; non-research workers, which are usually at lower level in an organization, typically are the problem-solvers.
If you're going to continue with the class, asking her about the framework used to answer questions might be helpful. You can't always find answers within a text, but you can learn how to methodically think through the questions. For example, when analyzing literature you can build a framework (getting background info on characters, historical context, author, symbols, themes etc.) and offer your argument or conclusions around them. You might be able to do the same with art.