To solve this question, the user needs to know about JSF expressions and how they can reference an application's context path.

Option A: #{contextPath} - This option is not correct. There is no JSF implicit object named contextPath. This option seems to be a custom attribute, which is not defined in the JSF specification.

Option B: #{requestContextPath} - This option is not correct. There is no JSF implicit object named requestContextPath. This option seems to be a custom attribute, which is not defined in the JSF specification.

Option C: #{facesContext.externalContext.requestContextPath} - This option is correct. This expression references an application's context path using facesContext.externalContext.requestContextPath. This expression is used to get the application's context path that the current request is mapped to.

Option D: ${request.contextPath} - This option is not correct. This expression is not a JSF expression. It is a JSP expression that is used to get the context path of the current request.

Therefore, the correct answer is:

The Answer is: C. #{facesContext.externalContext.requestContextPath}