Q.Which property of soft iron makes it useful for preparing an electromagnet?

[!TLDR] Soft iron has high permeability (so it magnetises strongly) together with very low retentivity (so it demagnetises almost completely once the current is switched off) -- exactly what a switchable electromagnet needs. [!ANSWER] High permeability with very low retentivity (and low coercivity).

An electromagnet (section 11.7) works by placing a ferromagnetic core inside a solenoid: passing current through the solenoid magnetises the core, boosting the field far beyond what the solenoid alone could produce, and the whole point of an ELECTROmagnet (as opposed to a permanent magnet) is that this magnetism must be switchable -- strong when the current flows, essentially gone when it doesn't. Soft iron is ideal for this because it has a large permeability (over 1000), so even a modest solenoid current produces a strongly magnetised core, while at the same time it has very low retentivity and low coercivity (a thin hysteresis loop), so almost none of that magnetisation survives once the current is switched off. If a hard ferromagnetic material were used instead, the core would remain magnetised long after the current stopped, which would defeat the purpose of a controllable, switchable electromagnet. [!ANSWER] High permeability with very low retentivity (and low coercivity).

Recall the electromagnet's requirement (section 11.7): a soft ferromagnetic core, chosen for high permeability with low retentivity/coercivity.

Thinking high retentivity would be desirable, on the reasoning that the electromagnet should "hold its magnetism strongly" -- that reasoning actually describes a PERMANENT magnet, and would make an electromagnet impossible to switch off.