MN1 - British Nationality Act 1981

My two children were born outside the UK. They are both having ILR since October 2010. I'm now planning to register them as British citizens using form MN1(because they are 16 & 13). I am a British citizen and they have been in the Uk with me since 2006.

On form MN1 number 1.1 'please indicate the section of the British Nationality Act 1981 under which you would like the application considered'.

Could someone help me which Act do i've to write in 1.1?Are my children qualify to be registered? If so under what category?

Thank you.

3(1).