Unclear how to submit a feature request, so posting it here in the hope it gets picked up.
I'm using Duo a lot on computer these days, and am liking the ability to use numbers to match the requested word (e.g. in Arabic: ' Select the correct character(s) for “mufa” ' provides four words, numbered 1, 2, 3, 4, with the relevant key press submitting your answer). This in tandem with pressing 'Enter' to advance the page along really works in speeding up exercises by avoiding the need for a mouse.
What I think would improve this approach is if in ' Match the pairs ' exercises, the options were similarly labelled (there are usually ten options, so the first nine could be numbered 1-9, the tenth could be e.g. ' x '). Sequential keyboard presses could be used to submit a match e.g. a ' 1 ', followed by a ' 5 ' would match the first and fifth words.