Option 1 states that only cotton contains nitrogen in its structure. This is incorrect because cotton does not contain nitrogen in its structure. Cotton is made up of cellulose, which is composed of carbon, hydrogen, and oxygen.

Option 2 states that both silk and wool contain nitrogen in their structure. This is correct. Silk is a natural protein fiber produced by insects and spiders. It contains nitrogen as a part of its amino acid structure. Wool, on the other hand, is made up of keratin, a protein that also contains nitrogen.

Option 3 states that only wool contains nitrogen in its structure. This is incorrect because, as mentioned earlier, silk also contains nitrogen.

Option 4 states that all three options – cotton, silk, and wool – contain nitrogen in their structure. This is also incorrect because cotton does not contain nitrogen.

In conclusion, the correct answer is option 2: Silk and wool both contain nitrogen in their structure.