Solution
Option 3: contribute to a syndrome called Multiple Chemical Sensitivity is correct.
The passage tells us that when a few harmful chemicals combine (mix together), they can lead to a condition called Multiple Chemical Sensitivity (MCS). This is a syndrome (a group of health problems that happen together) where a person's body becomes very sensitive to chemicals, even in tiny amounts. For example, someone with MCS might feel dizzy, get headaches, or feel sick just from smelling cleaning products or paint.
Why the other options are wrong:
Option 1 – The passage does not say that combining harmful chemicals causes allergies. Allergies are reactions to things like dust or pollen, which is different.
Option 2 – "A monumental task for homeowners" may appear in the passage, but it describes something else — not the result of chemicals mixing together.
Option 4 – Photosynthesis is how plants make food from sunlight. It has nothing to do with harmful chemicals combining.