How to check checkbox a pdf editor form using C#

This is my last hope please help me I am stuck with that check box in pdf editor. I'm using iText library to generate pdf file based on template. But when I try to set checkbox field it won't filled. My code is below

form.SetField("topmostSubform[0].Page1[0].c1_01_0_[0]","1");
            form.SetField("topmostSubform[0].Page1[0].c1_03_0_[0]", "true");
            form.SetField("topmostSubform[0].Page1[0].c1_02_0_[0]", "2");
            form.SetField("topmostSubform[0].Page1[0].c1_04_0_[0]", "On");

I have tried with following values 1) "Yes" 2) "1" 3) "true" 4) "On" 5) "2" some one suggest me pass this value. I tried but this one also not working in my code.

Which value should I pass to set check box. Please help me and I referred this link also but it is not working.