EASA learning objective 022.02.06.01.09 states: Define the term ‘unreliable airspeed’ and describe the means by which it can be recognised such as: different airspeed indications between ASIs; unexpected aircraft behaviour; buffeting; aircraft systems warning; aircraft attitude.

EASA learning objective 022.02.06.01.10 states: Describe the appropriate procedures available to the pilot in the event of unreliable airspeed indications: combination of a pitch attitude and power setting; ambient wind noise inside the aircraft; use of GPS speed indications and the associated limitations.

On multi crew aircraft the pilots ASI's are fed from different Pitot/Static souces and/or different air data computers (ADC's).  This can lead to slight variations in indication between instruments.  EASA CS 25 states that the difference between IAS/CAS indications must not exceed 3% or 5 knots whichever is the greater.  Most commercial air transport aircraft will display a caution alert if the discrepancy between ASI's exceeds this figure.  Therefore a ‘slight difference’ in indication can be considered to be within acceptable limits and no action neeeds to be taken.  

No, the lower airspeed is the most accurate --> INCORRECT. The lowest indication isn't necessarily the most accurate, ‘slight difference’ implies indications within limits

No, slight variations are normal --> CORRECT. ASI's fed from different pitot/static sources and/or ADC's will often have a slight difference in indication. CS 25 ASI limits are +3% or 5 kts whichever is the greater.

No, the crew should reset the Air Data Computer (ADC) --> INCORRECT. A slight difference is acceptable if within declared limits. The use of the word 'the' implies only one ADC, switching it off to 'reset' it will temporarily remove all air data infomation from the instrument displays!

Yes, the crew can cross-check groundspeed using the GNSS --> INCORRECT. A slight difference (less than 5kts) is inside limits and a gross error check such as this wouldn't resolve such a small error.

Note: The question states ‘slight difference’ which is implies within acceptable limits, there are similar questions where it's stated that the difference in indication is 5 kts or more which would be outside limits and produce a different answer.