LG 31MU97-B monitor support on rMBP 15"
I'm afraid not.

From Everymac


  *This model supports a simultaneous maximum resolution up to 2560x1600 on two external displays via Thunderbolt. Alternately, it can support a single display up to 2560x1600 via Thunderbolt and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.


From Apple


  You can use 4K displays and Ultra HD TVs at the following resolutions and refresh rates via the built-in HDMI port.
  3840 x 2160 at 30 Hz refresh rate
  4096 x 2160 at 24 Hz refresh rate (mirroring is not supported at this resolution)