$('select').on('change', function () {
        var subcategories = '';
        var str = "This category contains these subcategories: ";
        var result = str.bold();

        switch (this.value) {
               case 'Category 1':
                subcategories = "Subcategory 1.1, Subcategory 1.2";
                break;
                
                case 'Category 2':
                subcategories = "Subcategory 2.1, Subcategory 2.2";
                break;
                
                case 'Category 3':
                subcategories = "Subcategory 3.1, Subcategory 3.2";
                break;
                
                case 'Category 4':
                subcategories = "Subcategory 4.1, Subcategory 4.2";
                break;
 
        }
        document.getElementById("showSubcategory").innerHTML = result + subcategories;
    });