"De baby haalt de stekker uit het stopcontact."
Translation:The baby is taking the plug out of the outlet.
It's definition number 4 of 4 for a good reason: If you said outlet to an english speaking person in the UK they wouldn't have any idea that you meant a socket.
I followed up the link provided and found, in the "definitions for english learners" part of the Merriam Webster website, that "outlet" as a socket is an American term. A bit of Googling and it's clear that other forms of English (Australian, South African etc) also don't "outlet" to mean electrical socket.
Please accept socket as that's what most of the world uses!
Socket is accepted. While Duolingo uses American English as its 'preferred' translation, we are always careful to accept British or other regional alternatives. If not, feel free to send in a report.