Looking at the answers:

'Press the IDENT button and transmit; GOLF ALPHA CHARLIE BRAVO ROMEO HEADING' --> INCORRECT. There is NO ATC instruction involving the transponder.

'Transmit; GOLF ALPHA CHARLIE BRAVO ROMEO, TRANSMITTING FOR IDENTIFICATION PURPOSES, HEADING' --> CORRECT. By reporting the heading, you would already be transmitting for identification, which suggests this answer is over complicated, but it is always a safe technique to read back verbatim to ATC for these exams.

'Reselect Mode S and transmit; GOLF ALPHA CHARLIE BRAVO ROMEO HEADING' --> INCORRECT. There is NO ATC instruction involving the transponder.

'Set the transponder to STBY and transmit; GOLF ALPHA CHARLIE BRAVO ROMEO HEADING' --> INCORRECT. There is NO ATC instruction involving the transponder.

ICAO Doc 4444
12.4.1 General ATS surveillance service phraseologies

12.4.1.1 IDENTIFICATION OF AIRCRAFT

a) REPORT HEADING [AND FLIGHT LEVEL (or ALTITUDE)];
b) FOR IDENTIFICATION TURN LEFT (or RIGHT) HEADING (three digits);
c) TRANSMIT FOR IDENTIFICATION AND REPORT HEADING;
d) RADAR CONTACT [position];
e) IDENTIFIED [position];
f) NOT IDENTIFIED [reason], [RESUME (or CONTINUE) OWN NAVIGATION].

4.5.7.5 READBACK OF CLEARANCES
4.5.7.5.1 The flight crew shall read back to the air traffic controller safety-related parts of ATC clearances and instructions which are transmitted by voice. The following items shall always be read back:

a) ATC route clearances;
b) clearances and instructions to enter, land on, take off from, hold short of, cross, taxi and backtrack on any runway; and
c) runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and, whether issued by the controller or contained in automatic terminal information service (ATIS) broadcasts, transition levels.

4.5.7.5.1.1 Other clearances or instructions, including conditional clearances, shall be read back or acknowledged in a manner to clearly indicate that they have been understood and will be complied with.