Refer to figure.
Transition altitude → The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes using QNH.

Transition level → The lowest flight level available for use above the transition altitude. At or above the transition level 1013.2 hPa (QNE) pressure setting is used and the altimeter indicates flight levels.

Transition layer → The airspace between the transition altitude and the transition level. When climbing through this layer 1013.2 hPa setting is used and the altimeter indicates flight levels, and when descending through this layer QNH is used and the altimeter indicates altitudes.

When descending, once the aircraft reaches the transition level the altimeter setting should change from 1013.2 hPa (QNE) to QNH. Let's examine the given options:

Climbing through the transition level. → INCORRECT. At or above the transition level 1013.2 hPa (QNE) pressure setting is used.

Descending through the transition level. → CORRECT. When reaching the transition level during descent, the pressure setting should change from 1013.2 hPa to QNH.

Descending through the transition altitude. → INCORRECT. The transition altitude is the altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes using QNH. However, during descent the QNH is set earlier, when passing through the transition level.

ICAO Doc 8168
2.1.4 References to vertical position

2.1.4.2 Passing through the transition layer. 
While passing through the transition layer, vertical position shall be expressed in terms of:

a) flight levels when climbing; and
b) altitude when descending.