A – Wrong
The passage mainly focuses on TB and not weak immune systems. The conclusion must be something to do with TB. The statement may be true but it does not conclude the content of the passage. So this is wrong.
B – Wrong
Again, this statement only focuses on poor diet rather than the disease TB. This can be a reason for the increase in TB but not a conclusion of the passage.
C – Wrong
Seriousness and being unfamiliar with TB are two different things.
D – Wrong
It could be true but this is not a conclusion. It is important to stick with the content of the passage and the conclusion must contain a summary of what is in the passage. There is nothing written about overestimation in the passage.
E – Correct
The line “three quarters of those diagnosed in the UK every year were born outside the UK” helps us to identify the conclusion of the passage. Hence this is correct.