Final answer:
The word 'Lumen' refers to a space within a tube, such as the interior of blood vessels or the inside space of organelles like the endoplasmic reticulum and thylakoids in chloroplasts. The correct option is d.
Step-by-step explanation:
The correct definition of the word "Lumen" is d. A space within a tube. In biological context, a lumen typically refers to the inside space of a tubular structure in an organism, such as blood vessels or gastrointestinal tract. However, in the cellular context, it can refer to the inside space of cellular organelles such as the endoplasmic reticulum (ER) or thylakoid in chloroplasts.
For example, the luminal space is where protons accumulate during the light-driven electron transport in photosynthesis, and in the vascular system, it is the space through which blood flows. Hence, d is the correct option.