Be careful:

• The question asks for the "initial full call sign".
• The question states that the aircraft is a Boeing 747 - which is in the HEAVY wake category.

Looking at the answers:

'E-LINE ABC' --> INCORRECT. Missing 'Heavy' which is required on the first call

'European 941' --> INCORRECT. Missing 'Heavy' which is required on the first call

'747 Jumbo OBC Heavy' --> INCORRECT. 747 Jumbo? Use of the manufacturer's name is fine, but not the aircraft's nickname.

'Boeing OKABC Heavy' --> CORRECT. This meets all the requirements for an initial first call using a full sign. The name of the aircraft manufacturer (or aircraft model) can be used as a prefix to the full registration. More information from the regulation below.

ICAO Annex 10
Volume II - Chapter 5
5.2.1.7.2 Radiotelephony call signs for aircraft
5.2.1.7.2.1 Full call signs

5.2.1.7.2.1.1 An aircraft radiotelephony call sign shall be one of the following types:

Type a) The characters corresponding to the registration marking of the aircraft; or
Type b) The telephony designator of the aircraft operating agency, followed by the last four characters of the registration marking of the aircraft;
Type c) The telephony designator of the aircraft operating agency, followed by the flight identification.

Note 1: The name of the aircraft manufacturer or of the aircraft model may be used as radiotelephony prefix to the Type a) call sign.
Note 2: The telephony designators referred to in Types b) and c) are contained in Doc 8585 - Designators for Aircraft Operating Agencies, Aeronautical Authorities and Services.
Note 3: Any of the foregoing call signs may be inserted in field 7 of the ICAO flight plan as the aircraft identification. Instructions on the completion of the flight plan form are contained in PANS- ATM, Doc 4444.

Table 5-1 Examples of full call signs and abbreviated call signs