[karakym]

Чето получилось:

var arr=[
	{
		value: 6,
		list: [
			{
				value: 0,
				text: 'A3'
			},
			{
				value: 1,
				text: 'A4'
			},
			{
				value: 2,
				text: 'A5'
			}
		]
	},
	{
		value: 7,
		list: [
			{
				value: 0,
				text: 'M3'
			},
			{
				value: 1,
				text: 'M6'
			},
			{
				value: 2,
				text: 'X5'
			},
			{
				value: 3,
				text: 'X6'
			}
		]
	},
];

$("#category").change(function() {
selval = $(this).val();
forval = arr[selval - 6].list;
  $("#selectmodel").empty();
  $.each(forval, function(i, val) {
  $("#selectmodel").append("<option value='"+val.text+"'>"+val.text+"</option>");
})});
$("#selectmodel").change(function() {
text = $(this).val();
  $("#xf_model").val(text);
});