The only indication of giving corticosteroids in pustular psoriasis is –
Indications of systemic steroids in psoriasis

Generalized pustular psoriasis of pregnancy/erythroderma (only definitive indication)
Persistent, otherwise uncontrolled, erythrodernzic psoriasis causing metabolic complications.
Generalized pustular psoriasis, if other drugs are contraindicated or ineffective.
Severe psoriatic arthritis threatening severe irreversible joint dagmage (not in mild or moderate arthritis). 


Note - Some guides have given option d i.e. extensive lesions as answer. But they are wrong, following statement will clear all doubts :-"Systemic steroids are the drug of choice in pregnant women with exfoliative erythroderma (elythrodermic psoriasis) and generalized pustular psoriasis" .
 I have also confirmed it from many teachers of dermatology.

About extensive lesions 

Extensive lesions are called when greater than 30% of surface area is involved. Treatment of choice for extensive lesions is Narrow band UV-B incombination with emollient. Alternatives are PUVA, methotrexate, Acitretin, Cyclosporin, fumaric acid esters.
Psoriatic erythroderma with pregnancy
Psoriasis in a pt. with alchoholic cirrhosis
Moderate arthritis
Extensive lesions