Flexible round duct - What to use
While Code will let you use such a product, you need to be careful when doing so to avoid giving your HVAC system a case of the wheezes

I can find nothing in the 2015 or 2018 IRC that forbids the use of uninsulated flexible duct conforming to UL 181 Class 1 (the SDI limit in Code is 200) for HVAC supply or return air ducts located within the conditioned envelope.  However, there is a major caveat with this sort of product: it can put a real drag on airflow through it when installed improperly!

In particular, in order to correctly use flexible ducting in an HVAC system, you must stretch it all the way out when installing it, and support it so that it doesn't sag or collapse back on itself later, otherwise you will get excessive friction loss.  In other words, a sagging or unstretched flex duct restricts airflow through it, causing poor HVAC performance, and even excessive energy usage as many modern air handlers will fight to maintain airflow (CFM) despite the radically excessive static pressure needed to push air through the dodgy duct.