In Goldsmith thermite process, element undergoing oxidation is

According to electrochemical reactivity series, Al is more reactive than Fe. Hence, Aluminum (Al) undergoes oxidation in thermite process.

$2\mathrm{Al} +{\mathrm{Fe}}_2{\mathrm{O}}_3\to {\mathrm{Al}}_2{\mathrm{O}}_3+ 2\mathrm{Fe}$

$\mathrm{oxidation}: 2\mathrm{Al} \to 2{\mathrm{Al}}^{+3}+6{\mathrm{e}}^{-}$