What is the reasoning behind this question about the correct altitude to avoid clear icing?
As John K already said in the comments, 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,
  or
  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.