Option C is correct.
Initial is given as 20480 B, and minextents is given as 3,
So we have to allocate 3 extents (minextents is 3) intially to this file,
here first extent is 20480 B (because it is initial extent) and others two are Next extent (its size is also 20480 B, because no PCTincrease), PCTincrease is 0 (means percent of increase in next extent, here, no increase)
20480(initial) + 20480*2(Next extent) = 20480*3=61440Bytes