I wish you had included as options "both index finger and thumb" and "index finger and thumb together" instead of, or along with, "depends" and "other" because both of those options could suggest something different from either index finger or thumb. I selected "depends." I was strumming with my index finger until I broke my middle finger on my strumming hand. While my finger was splinted and for a while afterwards while I regained mobility in it, I strummed with my thumb because I couldn't roll my fingers other than my index finger into a fist and holding my hand open while trying to use my index finger was painful. Once my finger was back to almost normal (it healed a little crooked), I had trouble converting back, and I had become proficient with my thumb. Now, I tend to strum with my index finger when I play Hawaiian music, although some strums require me to use both my index finger and my thumb, and with my thumb when I'm playing popular music.