Welcome to Android Central! Which HTC One do you have? There were numerous models and generations carrying that name.
Most phones require you to swipe the screen to answer a call, to prevent unintentional answering (like if the phone was in your pocket). Some phones might have an Accessibility option that allows for one-touch answering, but we'd have to know which exact phone you have.
As an aside, whichever HTC One phone you have is pretty old and outdated by now, and has not received any Android security update for many years. From a security and privacy standpoint, you should strongly consider upgrading to a more current phone.
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