$("#btnAgregarFila").click(function () {
        var cont_fila = ($('#det_comprobante tbody').find('tr').length) + 1;
        var fila = '<tr>';
        fila = fila + '<td scope="row">' + cont_fila + '</td>';
        fila = fila + '<td><input type="text" class="form-control text-center" id="respuesta_' + cont_fila + '" placeholder="Respuesta" "/></td>';
        fila = fila + '<td><select class="form-control text-center" id="alternativa_' + cont_fila + '" "><option value="">.:: ELIJA ::.</option><option value="1">SI</option><option value="2">NO</option></select></td>';
        fila = fila + '<td><a href="javascript:void(0);" class="eliminar_fila" onclick="eliminar_detalle(this,' + cont_fila + ');"><i class="fa fa-trash-o fa-2x"></i></a></td>';
        fila = fila + '</tr>';
        $("#det_comprobante").append(fila);
    });

function guardar_pregunta() {
        var detalle = "[";
        for (var i = 1; i < document.getElementById('det_comprobante').rows.length; i++) {
            detalle = detalle
                    + '{"respuesta":"'
                    + document.getElementById('det_comprobante').rows[i].cells[0].childNodes[0].value + '",'
                    + '"alternativa":"'
                    + document.getElementById('det_comprobante').rows[i].cells[1].childNodes[0].value + '",'
                    + '}'
            if (i == document.getElementById('det_comprobante').rows.length - 1) {
                detalle = detalle + "]";
            } else {
                detalle = detalle + ',';
            }
        }
        console.log(detalle);
    }

[{"respuesta":"undefined","alternativa":"Hola",},{"respuesta":"undefined","alternativa":"undefined",}]