a. 144,000 is Literal (as in)
b. 144,000 is Symbolical, but it is different from Great Multitude
c. 144,000 is Symbolical, and part of the Great Multitude
d. 144, 000 is the Symbolical number of the Great Multitude
Attached is a study that I did on this topic. I hope that you enjoy reading it.