This is googles fall back method if it is unsure if you are a bot
It will appear for some users, if they haven't triggered enough "human activity"
It generally doesn't pop up if you are signed in with google and have completed reCaptcha in the past
There are no options to turn this off, and you wouldn't want too, since it appears when big G is unsure (mostly for bots, not so much for humans that have "proved their selves")