Storyline 360 - Is there a way to make all answers to questions tab-able?

The reason it works this way is because they are part of a Button Set, which is basically a group of selectable objects where only one object can be selected at a time. Once they get into that group, navigating with up/down arrows would be the user's expected behavior.

You could select all of the answers on your slide, right click on them, and remove them from their automatically assigned "Pick One" button set. However, this removes the functionality that allows the user to only select one object at a time, via mouse or keyboard. They could select one, then select another, and they'd both be selected (as opposed to button set behavior, which would automatically deselect Item 1 when they selected Item 2). They would have to go back and deselect the first item they selected. But, this would let them use the Tab key between each answer option. From a learner perspective, I'm not sure that's better. You'd also want to test how they would go about de-selecting an item via keyboard!