Given this is the private flying forum, I'd assume that OAT=IOAT
A quick bit of back of envelope algebra by the way tells me that (within about 3%):
TAS = IAS / SQRT ((288 * air pressure) / ( Absolute Temperature * sea level air pressure))
[clarifying, TAS = EAS/SQRT(sigma); sigma is relative pressure = rho/rho_0, but I just worked it out with temperature and pressure. Below 0.8 Mach and FL100 it's reasonable to assume that EAS=CAS, and certification requirements permit a maximum of 5% error between IAS and CAS.]
In this case however you've got altitude, mach number and temperature. Dead easy.
Speed of sound in metres per second, c = SQRT (gamma * R * T)
Gamma = 1.4 (always is because air is a diatomic gas)
R = 287 J/kg.K (constant for air)
T = 273.15 - 15 = 258.15K
So c = 322 m/s
So, at M=0.4, TAS = 0.4 * 322 = 129m/s
None of your answers are in metres per second, but 1 knot = 0.514 m/s, so let's try that. 129 / 0.514 = 250 knots.
Bingo: 250 knots is your answer.
G