Ctrl Alt Delete on RDP with Macbook installed windows 7

This is what worked for me on my Macbook Pro using Yosemite (10.10.2) and the official RDP client from Microsoft while connected to a Windows 2012R2 server:

fn+control+alt+delete

I found it, it was:

fn+ctrl+alt+right-arrow

I cannot comment on other answers because of my "reputation points". What worked for me was slm but in this format:

fn+control+option+delete

MAC keyboard.