According to the textbook context, 'A passage number refers specifically to the number of times a cell line has been sub-cultured.' The text further clarifies that 'Fig: 8.6: Step involved in Subculture/Passage' directly equates subculture with passage. The process involves detaching cells from the culture vessel using enzymatic or mechanical means and transferring them into fresh media. Therefore, passaging is synonymous with sub-culturing of cells, making option (A) the correct answer.