Which sub-scale setting may a pilot use on an instrument approach in South Africa--QFE or QNH?
I have never been given a QFE to conduct an instrument approach in S.A. It does happen in some countries but then everyone has to use it. If you look on an approach chart you will see the vertical profile is given in terms of height above sea level so you wouldn't be able to conduct the approach with QFE set. Edit: The SACAA charts do show height of the waypoints of the vertical profile, but Jeppesen doesn't. The MDA and DH are given terms of elevation and height, but this is to use in conjunction with the radio altimeter. QFE might be used by aerobatic pilots or some military operations but I think its use is rare. Having said that, the AIP does appear to allow the use of QFE on a second altimeter during the final approach see note(2) and point (f) below. Therefore it appears that B is the answer the CAA is looking for. Using the word &quot;only&quot; in answer A would make it incorrect.
Please look at the AIP here with particular attention to the Altimeter Setting Procedures, I will try elaborate in due course.
Firstly the AIP says

The altimeter setting procedures in use conform to those contained in
ICAO Doc 4444 ATM/501 Procedures for Air Navigation Services and ICAO
Doc 8168 OPS/611 Procedures for Air Navigation Services - Aircraft
Operations

Then it says

During flight in the vicinity of an aerodrome at or below a fixed
altitude called the transition altitude, an aircraft shall be flown at
altitudes determined from an altimeter set to sea level pressure (QNH)
and its vertical position shall be expressed in terms of altitude.

Further down below it says

On reaching the transition level at least one altimeter within the
aircraft shall be set to the QNH and thereafter the vertical
positioning of such aircraft shall be by reference to altitudes.
Note 1: On reaching the transition level, pilots shall reset their
altimeters to the QNH without requesting Air Traffic Control (ATC)
permission to do so and without notifying Air Traffic Control (ATC)
that the change has been made.
Note 2: This does not preclude a pilot from using a QFE setting for
terrain clearance purposes during the final approach to the runway.
f)   During the approach to land, terrain clearance may be determined
by using either a QNH altimeter setting (giving altitude) or a QFE
setting (giving height above the QFE datum)

I believe the intention is to allow the pilot the discretion of using a second altimeter with the QFE set, but when you talk to ATC it will be with reference to the altimeter having QNH set.