In this question, it is easy for us to spot the opening sentence. It brings the action from the past (1990s) into the present. For this reason, 3i in the present perfect continuous form, and 1 is in the simple present form. Thus 31 form a pair. 4 has the pronoun "these concerns". It refers to the noun "pressing security concern" in 2. Thus 24 form a pair. The right sequence has to be 3124 .