The passage given below is followed by four summaries. Choose the option that best captures the essence of the passage.

Zombie cells may contribute to age-related chronic inflammation: this finding could help scientists understand more about the aging process and why the immune system becomes less effective as we get older. Zombie or “senescent” cells are damaged cells that can no longer divide and grow like normal cells. Scientists think that these cells can contribute to chronic health problems when they accumulate in the body. In younger people, the immune system is more effective at clearing senescent cells from the body through a process called apoptosis, but as we age, this process becomes less efficient. As a result, there is an accumulation of senescent cells in different organs in the body, either through increased production or reduced clearance by the immune system. The zombie cells continue to use energy though they do not divide, and often secrete chemicals that cause inflammation, which if persistent for longer periods of time can damage healthy cells leading to chronic diseases.

The passage describes how ageing, a weaker immune system, and chronic disease are connected. It introduces senescent or “zombie” cells, explains that younger people clear them more easily, and shows that this process slows with age. As a result, these cells build up, causing ongoing inflammation and damage to healthy tissue. Based on this, we consider each option individually. 

Option A is too limited. It correctly says that senescent cells are malfunctioning and present in the body, but it misses the main point: as people age, these cells are not cleared as well, they build up, and this leads to inflammation.

Option B points out the differences between younger and older immune systems and mentions apoptosis, but it wrongly states that zombie cells 'resist' apoptosis. The passage actually focuses on the immune system becoming less effective, not the cells resisting. It also leaves out the important role of inflammation and chronic disease.

Option C covers the whole chain described in the passage. It links ageing to less effective apoptosis, explains how zombie cells build up, and includes their role in inflammation and chronic disease. This matches both the structure and the focus of the original text.

Option D is incorrect and somewhat out of the scope of the passage. The passage is about metabolically active senescent cells, not dead cells, and it does not say that inflammation directly weakens the immune system as this option suggests.

So, option C best sums up the main idea of the passage.