Answer:
Content validity
Step-by-step explanation:
Content validity refers to the fact that a study actually measures all aspects of the construct.
For a study to be valid, the measurement method must cover all relevant parts of the subject it is measuring. If it doesn't do so, then the validity of the study is treatment because of content validity.
In this case, the construct we are studying (quality of a relationship) has three parts (belong, interdependence and intimacy). Since the questionnaire doesn't include items tapping intimacy it is not covering all three parts of the construct and therefore it is lacking content validity.