B

A 4-g IV loading dose is recommended in standard MgSO₄ regimens for eclampsia prophylaxis. Since 50 % w/v MgSO₄ contains 500 mg/mL, 8 mL provides 4 g (8 mL × 500 mg/mL = 4000 mg). Diluting this with 12 mL normal saline yields 20 mL of a 20 % solution suitable for slow IV infusion, matching option B [Sibai, 1984, PMID 6496595; Lu & Nightingale, 2000, PMID 10803454].