Previous question button

Firstly I would refactor out your code a bit.

I would create a method called "RetrieveQuestion" that accepts an integer to indicate which question it is getting.

int RetrieveQuestion(int questionNumber)
{
SqlConnection con1 = new SqlConnection(connection);
con1.Open();
SqlCommand cmd = new SqlCommand("Select * from quiz1 where id=" + questionNumber + " ", con1);
SqlDataAdapter data = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
data.Fill(ds);
DataView dv = ds.Tables[0].DefaultView;
 
if (dv.Count > 0)
{
string id = dv[0]["id"].ToString();
string question1 = dv[0]["question"].ToString();
string c1 = dv[0]["c1"].ToString();
string c2 = dv[0]["c2"].ToString();
string c3 = dv[0]["c3"].ToString();
 
question.Text =question1; // displaying question here
RadioButton1.Text = c1; // option 1
RadioButton2.Text = c2; // option 2
RadioButton3.Text = c3; // option 3

return questionNumber +1;
}

return questionNumber; 

}

Then in the Next button event I would have:

protected void next_Click(object sender, EventArgs e)
{
i = RetrieveQuestion(i);
}

and, because RetrieveQuestion returns i+1 in the Previous Button event I would have:

protected void previous_Click(object sender, EventArgs e)
{
i = RetrieveQuestion(i-2);
}

This is because i is currently the next question, not the current question.