The correct option is A whom
In the sentence, the object ("the journalists") is acted upon ("had invited") by the subject ("he"). The sentence is a combination of two sentences that have been joined together 'He had invited the journalists for his press conference. Very few of the journalists turned up for his press conference'. These are combined by a relative pronoun, that functions as a pronoun for the noun "journalists" as well as a conjunction as it links the sentences. From the options, 'whom' is used because it refers to the journalists while combining the sentences, hence option A is correct. 'Who' is generally used in informal English, whereas 'whom' is used in formal English, hence option B is incorrect. 'Whose' is a possessive pronoun, it shows possession of something, for example, 'The journalist whose article is on the front page...', 'whose' shows possession of the article by the journalist. In the given sentence, there is no possession indicated by the journalists, hence option C is incorrect. 'Which' is generally used for things, whereas here a relative pronoun is required to refer to persons "journalists", and not things. Hence option D is incorrect.