Question: In 3GPP RRC protocol specification, a field declared with the `OPTIONAL` keyword in ASN.1 notation implies that:
Option 1: The field must always be present but can have a null value.
Option 2: Omission of the field carries specific semantics different from any normal value of the field.
Option 3: The field's presence is mandatory, but its value can be defaulted if not specified.
Option 4: The field is deprecated and should not be used in modern implementations.
Explanation: In the context of 3GPP RRC protocol specification using ASN.1 notation, declaring a field with the `OPTIONAL` keyword signifies that the absence (omission) of this field conveys particular meaning or semantics, distinct from any value the field might normally hold when present.