IANYL. Microsoft owns a trademark registration for X [standard character] for certain goods and services (see: https://tsdr.uspto.gov/#caseNumber=2693757&caseSearchType=US... ). Meta owns a trademark registration for X (Stylized), which Microsoft apparently assigned to Meta (see https://tsdr.uspto.gov/#caseNumber=87980831&caseType=SERIAL_... ). Musk as been using X in various capacities since the 90s, and depending o…
Honestly Apple probably has the best shot, as they do use "X" prominently. Mac OS X, iPhone X, etc.