Refer to figure.
EASA learning objective 022.02.04.01.10 states: 'Describe the use of GPS altitude as an alternative means of checking erroneous altimeter indications, and highlight the limitations of the GPS altitude indication.'

An Air Data Computer (ADC) processes air data such as static and total pressure and outputs it to the electronic air data instruments. EFIS equipped aircraft are required to have standby instruments which use their own pitot/static sources in case of ADC failure. An alternative source of altitude information is from Global Navigation Satellite Systems such as GPS. GNSS systems calculate height above the surface of a mathematical model of the Earth. This geodetic model is a smoothed flattened sphere so the altitude output is differs from the Mean Sea Level reference of barometric altimeters. However, provided that satellite coveage is good GNSS altitude can provide a useful reference.

radio altimeter --> INCORRECT. The normal working range for a radio altimeter is 0 - 2500 ft.

transponder --> INCORRECT. If the ADC has failed the transpnder won't receive any barometric altitude information and it's not displayed on the flight deck anyway.

GPS --> CORRECT. GNSS can provide geometric height or altitude information.

weather radar --> INCORRECT. Weather radar does not provide altitude infomation.