var select = document.createElement("select");
var option = document.createElement("option");
select.setAttribute("onchange", "mod_auswahl\(\)");
select.appendChild(option);

document.getElementById('tabellenzeile').innerHTML = '';
document.getElementById('tabellenzeile').appendChild(select);

select.options[0] = new Option('A','A',true,true);
select.options[1] = new Option('B','B',false,false);
select.options[2] = new Option('C','C',false,false);