They are those who were chosen by God from before the foundation of the world unto salvation. He chose them because He decided to save those specific individuals, but not everyone, and not because He knew in advance they would later do something to deserve being saved - that would make salvation by their works and not by grace. The elect, by their nature, until born-again, are every bit as sinful as the non-elect are and no more deserving of salvation than they are. They obtain and demonstrate the attributes of true Christians from becoming saved/born-again, not to become so.
God chose those of the elect because it was His divine prerogative and good pleasure to arbitrarily choose whomsoever He would, and He demonstrated that through the unilateral actions of His choices. Being God, He owes no explanation or apology to anyone because He did so. He makes that point clear in the verses below.
[Rom 9:13-24 KJV]
13 As it is written, Jacob have I loved, but Esau have I hated.
14 What shall we say then? [Is there] unrighteousness with God? God forbid.
15 For he saith to Moses, I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion.
16 So then [it is] not of him that willeth, nor of him that runneth, but of God that sheweth mercy.
17 For the scripture saith unto Pharaoh, Even for this same purpose have I raised thee up, that I might shew my power in thee, and that my name might be declared throughout all the earth.
18 Therefore hath he mercy on whom he will [have mercy], and whom he will he hardeneth.
19 Thou wilt say then unto me, Why doth he yet find fault? For who hath resisted his will?
20 Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed [it], Why hast thou made me thus?
21 Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?
22 [What] if God, willing to shew [his] wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction:
23 And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory,
24 Even us, whom he hath called, not of the Jews only, but also of the Gentiles?