Zhenyaxxxx,
от вас требовалось содержимое строк 34 - 37 ... !!!
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.diametr_form label{
display: table-row;
}
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
var checkbox = $('[name="diametr"]'), block = $(".prom_vents~#product");
block.hide();
checkbox.change(function() {
var cls = "" + $.map(checkbox.filter(":checked"), function(el) {
return '[class~="'+el.value+'"]'
});
block.hide().filter(cls).show();
});
});
</script>
</head>
<body>
<form method="post" action="" class="diametr_form"><label></label>
<label><input type="checkbox" name="diametr" value="100" class="d100" />100 мм</label>
<label><input type="checkbox" name="diametr" value="125" class="d125" />125 мм</label>
<label><input type="checkbox" name="diametr" value="150" class="d150" />150 мм</label>
</form>
<h2 class="prom_vents">вентиляторы</h2>
<div id="product" class="100">100</div>
<div id="product" class="125">125</div>
<div id="product" class="150">150</div>
</body>
</html>