Salicylic acid also called the 2-Hydroxybenzoic acid is a benzene ring that has an alcohol and a carboxylic acid in an ortho position.
Upon the nitration of the salicylic acid, para product is the major product. The key in learning this is the mode of directions of the substituents of in the benzene ring. The -OH group is an ortho and para director and the C(=O)-OH is a meta director. It is hard to explain it here but please bear with me.
If the -OH is in the carbon 1, it will direct it either to its ortho (carbon 2 and carbon 6) or para (carbon 4). Since ortho connection is sterically unfavorable, the nitration is happening to the para group (carbon 4).
Now, the C(=O)-OH is in the carbon 2. It is meta directing and therefore will go to carbons 4 and 6. Since we already know that the carbon 6 is sterically hindering, the carbon 4 attachment is the best way.