Here it is
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QuoteThe R.M.S. speed of the molecules of a gas of density 4 kg m-3 and pressure 1.2 x 10⁵N m· 2is:
(A) 120 ms·' ( 300 ms·' (C) 600 ms·' (0) 900 ms·'

µ= √(3RT/M)-Root mean square velocity

PM=dRT (ideal gas equation)
M= dRT/P
Plug in this in root mean square velocity equation

µ= √(3RT P/dRT.   )

µ= √(3P/d.)

Now plug in the value of P and density in SI units
in the question, the value of pressure and density is already given in assignments so there is no need of any conversion.