For a simply supported beam of span 15m, the minimum effective depth to satisfy the vertical deflection limits should be

750mm

For >10m span,, span/depth = span^2/22x10=15x15/22x10=1.125>1m

if we have span greater than 15m then formula be like or you may can say verticle deflection may generally be satisfied as fallows
span2*2 ok...............
if we have span less than 10m then verticle deflection limit may satisfied as fallows
for cantiliver=7
for simply supported=20
for Continious=26

Ans should be..more than 1 m

Answer should be d
As Length is more than 10; which is 15
l/d = 20*(10/15)
d = (15*15)/(20*10) >1 m