Given,

The mean of 12 observations is 30

Sum of observation= 30×12 = 360

By error, one observation was noted as 12, but was misread as 21

(Correct mean= Sum of observation+ correct number- incorrect number)

The correct mean

= 360+12-21= 351

∴ Required average of these numbers is

= 51÷5= 29.25

Hence, Option A is correct.