Per the documentation, the method signature for select is:
select(object, method, choices, options = {}, html_options = {})
html_options, which is where your HTML attribute options like :class should go, comes after options, but you're omitting options and putting html_options right after choices. Try this instead:
<%= f.select :customer_type, options_for_select(["Retail","Contractor","Dealer"]), {}, class: "form-control" %>
P.S. If you're wondering why the method signature specifies object, but in actual use we never pass object (method is always first), it's because we're not actually calling this method directly. When we call f.select(...) inside a form_for block, Rails calls select(f.object, ...) for us.
