The crystal structure of λ iron (austenite phase) is

The correct option is FCC.

Based on the cooling curve of pure iron shown in the provided diagram (plotting Temperature against Time), we can track the structural changes of iron at different temperature thresholds:
1. Delta Iron (
$\delta$
-iron): Liquid iron solidifies at
$1538\text{°C}$
to form
$\delta$
-iron. As labeled in the diagram under "δ - icon (BCC)", this phase has a Body-Centered Cubic (BCC) crystal structure.
2. Austenite Phase (
$\gamma$
-iron / gamma iron): Upon cooling below
$1394\text{°C}$
down to
$912\text{°C}$
, iron undergoes an allotropic transformation into the austenite phase (labeled as "γ - icon (FCC)" in the diagram, which corresponds to the
$\lambda$
-iron typo in the question text). The crystal structure of austenite is Face-Centered Cubic (FCC).
3. Ferrite Phase (
$\alpha$
-iron): Below
$912\text{°C}$
, the structure reverts to a Body-Centered Cubic (BCC) crystal structure, labeled as "α - icon (BCC)".

Furthermore, the diagram indicates a magnetic transition at
$770\text{°C}$
(the Curie temperature), below which the iron transforms from paramagnetic to ferromagnetic.