This is not Artoria Pendragon since she has blue clothes, big boobs, no hair bun, and she is the king. This is Saber since she has blue clothes, small boobs, a hair bun, and she is not the king yet. There are other mainline sabers like Mordred (Saber of Red) ~red clothes, pony tail small boobs~ and Nero Claudius (Red Saber) ~red clothes, hair bun big boobs~, but we'll ignore that for now.
Once you figure it is Saber, you need to narrow it down to 1 of 5 sabers: 1. Saber 2. Saber (Alter) 3. Saber (Archer) 4. Saber (Lily) 5. Saber (Rider Alter)
And then when you narrow it down to Saber (Rider Alter) there are 5 version 1. Saber (Rider Alter) - Maid 2. Saber (Rider Alter) - Stage 1 3. Saber (Rider Alter) - Stage 1 variant 4. Saber (Rider Alter) - Stage 2 5. Saber (Rider Alter) - Stage 3 6. Saber (Rider Alter) - Stage 4
This is highly confusing and really hard since I don't even play the Fate Grand Order game. I just like labelling things and putting pictures in file folders with proper names.