ICAO Doc 4444 
4.10.4 Provision of altimeter setting information

4.10.4.3 The flight crew shall be provided with the transition level in due time prior to reaching it during descent. This may be accomplished by voice communications, ATIS broadcast or data link.

4.10.4.4 The transition level shall be included in approach clearances when so prescribed by the appropriate authority or requested by the pilot. 

4.10.4.5 A QNH altimeter setting shall be included in the descent clearance when first cleared to an altitude below the transition level, in approach clearances or clearances to enter the traffic circuit, and in taxi clearances for departing aircraft, except when it is known that the aircraft has already received the information.

Unlike the transition altitude, which is normally fixed for a given aerodrome, the transition level is variable because it depends on the current atmospheric pressure (QNH). For that reason, pilots cannot simply memorize it before departure; they must be informed of the current transition level before reaching it during descent.

The regulation therefore requires that the flight crew receive the transition level in due time prior to descending through it, ensuring sufficient time to prepare for the change from standard pressure (1013.2 hPa) to the local QNH.

The regulation also specifies that this information may be provided through:

• ATC voice communications,
• ATIS broadcasts,
• or data link services.

Therefore, the correct answer is: By ATIS and within sufficient time before descending through the transition level.

As for the other incorrect options:

At any altitude, by VOLMET ---> VOLMET provides meteorological information to aircraft in flight. It is not used to provide the transition level.

At 2500 ft, by data link ---> The transition level is relevant before descending through the transition layer, typically long before reaching such a low altitude. Waiting until 2500 ft would be far too late.

At 3000 ft, by ATC ---> The regulation does not tie the provision of the transition level to a specific altitude. The requirement is that it be provided in due time before reaching the transition level, not at 3000 ft.