The correct option is C Has been built
Here, 'has been built in present perfect continuous is correct because it is used to
indicate an action that started at some point in the past and may or may not be
complete. Here, the construction started at some point of time in the past by the best engineers.
'Had been built' in past perfect continuous is
incorrect because it is used to indicate an action in the past that began
before a certain point in the past and continued up untill that time.
'Was built' in past continuous is
incorrect because it is used to indicate uncompleted action in the past (with
or without time reference).
'Is built' in past participle is incorrect. The past participle is used with the verb have (have / has / had) to create the present and past perfect tenses. The past participle form is also used to modify nouns and pronouns.