Укажите лишнее..a.trousers, b.jeans, c.tights, d.skirts

Skirts (юбки) - лишнее, потому что это существительное во множественном числе. остальные существительные в единственном числе не существуют :
trousers (брюки), jeans (джинсы), tights (колготки)

Trousers - брюки
Jeans - джинсы 
Tights - колготки
Skirts - юбки 
Думаю что лишнее: C) Tights - колготки
Так как оно идёт под низ, а все это идёт на верх :)

1. You know the elderly person living on the 7th floor? 2. Who has bought the big vase standing on a floor in your room? 3. The people smoking at office have to leave immediately. 4. Who these people who are shaking hands each other? 5. The colleague trying to help me, my best friend. 6. You like the people thinking more of the future, than of the present? 7. You read article about the experts coming today to Moscow? 8. Why the managers discussing this question don't agree with each other? 9. The police officers investigating this crime haven't found the murderer yet. 10. The boy all the time preventing us from working, the son of our secretary. Вроде так

1. You know the elderly person living on the 7th floor? 2. Who has bought the big vase standing on a floor in your room? 3. The people smoking at office have to leave immediately. 4. Who these people who are shaking hands each other? 5. The colleague trying to help me, my best friend. 6. You like the people thinking more of the future, than of the present? 7. You read article about the experts coming today to Moscow? 8. Why the managers discussing this question don't agree with each other? 9. The police officers investigating this crime haven't found the murderer yet. 10. The boy all the time preventing us from working, the son of our secretary. Вроде так