Antecedent of pronoun 'them' can be decided with the help of 'within' before 'them' in option E.

As per rule, I agree that them could refer to nearest preceding plural noun. However, if we refer to tourists sentence becomes illogical.

Another reason for chosing E is usage of 'because of'.

Hope it helps.

Thank for the reply. But, I am still confused as if we will read the whole sentence ten withih them seems related with tourists. Big time confused I am. could you please elaborate the explanation of could you please in some other way. It will be a great help.

Thanks

Lets rewrite the original question by plugging in option E into it.

In late 1997, the chambers inside the pyramid of the Pharaoh Menkaure at Giza were closed to visitors for cleaning and repair because moisture exhaled by tourists had raised the humidity within them to such levels that salt from the stone was crystallizing and fungus was growing on the walls.

Lets replace them with tourists and rewrite

In late 1997, the chambers inside the pyramid of the Pharaoh Menkaure at Giza were closed to visitors for cleaning and repair because moisture exhaled by tourists had raised the humidity within tourists to such levels that salt from the stone was crystallizing and fungus was growing on the walls.

Lets replace them with chambers and rewrite

In late 1997, the chambers inside the pyramid of the Pharaoh Menkaure at Giza were closed to visitors for cleaning and repair because moisture exhaled by tourists had raised the humidity within chambers to such levels that salt from the stone was crystallizing and fungus was growing on the walls.

You notice the difference? humidity within tourists does not appear to make sense while humidity within chambers does.

Hope it helps.

Now, it is crystal clear..... Thanks for the help. The explanation was awesome. You gave me a new way to think. Thanks