In one of the more intuitive main lines following , after Black's defense with at a, White switches to the top left.
The option provided in this exercise leads to a similar but more peaceful variation, with moving into the centre and Black switching to the top left.
If Black plays the top left, which LZ would do low rather than high, White defends and Black launches the taisha!
The other two options, a and c lose respectively 7,5% and 11% with respect to the top choice and can be considered mistakes. In both cases White will be happy to play b and take the lead.