Explanation

Security arrangements have been tightened up in all sensitive areas.

Sensitive areas حساس علاقے

sensitive (Adj.) means: readily or excessively affected by external influences

Here, sensitive is the right usage.

Explanation

 They is a subject pronoun, and them is an object pronoun.

1) They

They is used to refer to the subject of a clause. In other words, it usually represents the ‘doers’ of the action described by the verb, and usually refers back to two or more people or things that were mentioned earlier:

2) Them

Them is used to refer to the object of a clause. In other words, it usually represents the group of people or things that have ‘experienced’ the action described by the verb, and refers back to two or more people or things that were mentioned earlier: