Hi
Please remove the single quotes from the query and try.
string a = comboBox1.SelectedItem.ToString();
string a1 = comboBox2.SelectedItem.ToString();
groupBox3.Visible = true;
da = new SqlDataAdapter("select " + a + "," + a1 + " from probablitystate1", cn.con);
dt = new DataTable();
da.Fill(dt);
dataGridView1.DataSource = dt;