The definition for icing conditions is visible moisture at a TAT of less than 10°C. From the Boeing 737 NG FCOMv1 (SP.16.1 Supplementary Procedures - Adverse Weather):
Icing conditions exist when OAT (on the ground) or TAT (in-flight) is 10°C
or below and any of the following exist:
- visible moisture (clouds, fog with visibility less than one statute
mile (1600m), rain, snow, sleet, ice crystals, and so on) is present,
- ice, snow, slush or standing water is present on the ramps,
taxiways, or runways.
To avoid icing, you should descend to an altitude where the temperature is above 10°C. The standard lapse rate of the atmosphere according to the International Standard Atmosphere (ISA) is 2°C per 1,000 ft:
As an average, the International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 K/km (3.56 °F or 1.98 °C/1,000 ft) from sea level to 11 km (36,090 ft or 6.8 mi).
This would result in the following expected temperatures at the given altitudes:
- A) 14,000 ft: -2°C
- B) 12,000 ft: 2°C
- C) 6,000 ft: 14°C
- D) 9,000 ft: 8°C
This only leaves answer C) with a temperature above 10°C and therefore outside of icing conditions.
Note: The lapse rate of 2°C per 1,000 ft is only an average, the actual lapse rate could be somewhat different. Wikipedia says:
A typical value is around 5 °C/km, (9 °F/km, 2.7 °F/1,000 ft, 1.5 °C/1,000 ft).
Maybe the question assumes a lower lapse rate of 1°C per 1,000 ft, where an altitude of 6,000 ft is exactly where you would get 10°C.