Explanation:

 Wet deposition of acid occurs when any form of precipitation (rain,snow and so on) removes acidic pollution from the atmosphere and delivers it to earth's surface

The acidic pollution can be formed due to oxides of sulphur  and nitrogen which will be in larger amounts suspended in the atmosphere  and get washed away    in large amounts after the first rain. In later spells of rain ,lower and lower quantity may be present which is washed away as acid rain. So, (a) will be correct.

Terpenes are a large and diverse class of organic compunds ,produced by a variety of plants particularly conifers.  They are not relevant here. So, (c) is incorrect