The crystal structure of λ iron (austenite phase) is

The correct option is FCC.

Step-by-Step Explanation:
Iron is allotropic, meaning it changes its crystal structure at different temperatures. By examining the temperature-time cooling curve of pure iron provided in the image, we can identify the following phases:
1. α-iron (Alpha phase / Ferrite): Below 912 °C, iron possesses a Body-Centered Cubic (BCC) structure, labeled in the diagram as "α - icon (BCC)". It is ferromagnetic below 770 °C and paramagnetic above it.
2. γ-iron (Gamma phase / Austenite): In the temperature range of 912 °C to 1394 °C, iron transitions into the austenite phase, which has a Face-Centered Cubic (FCC) structure. In the diagram, this is shown as "γ - icon (FCC)" in the paramagnetic region. Note that the symbol "λ" in the question is a typo for "γ" (gamma) iron.
3. δ-iron (Delta phase): Above 1394 °C up to the melting point of 1538 °C, iron reverts to a Body-Centered Cubic (BCC) structure, labeled as "δ - icon (BCC)".

Consequently, the crystal structure of austenite (γ-iron) is FCC (Face-Centered Cubic).