What's going on here?

The clues we have are:
- angry veins on her hand
- sweat beads on the guy's face
- embarrassed face on the guy

From my deductive power I'm guessing:

1. He used the sock to clean up semen and she found it.
2. She found her sock in his room.
3. He tried to shoplift a sock from a store and she caught him.
4. He is a dirty boy who doesn't wash his socks and she is scolding him.

whatever it is...

somehow yukiko looks awesome here o_o

No! that is chie 's sock

Taken from the log:
「この靴下誰のかしらね？」
「ナ、ナナコチャンノダヨ？」
「それはそれで問題ね…」
「ﾋｨ!」

- I wonder whose socks are these~ *irony*
- Na-Nanako-chan's, aren't they? *sweat*
- That being the case that's a problem, huh... *burrows finger into his chest*
- Problem indeed..

I don't know their background, or anything about the characters, but it sounds like Jealousy.

[Correct me if my japanese is broken]

There is a scene in P4 where Naoto talks about losing a sock in your room...

I can see why she'd be suspicious; why would Nanako have a sock with the Yasogami High logo?