# Which of the following is Lewis base: AlCl3, CO, CO2, or BF3?

$\text{CO}$
${\text{AlCl}}_{3}$ , ${\text{BF}}_{3}$ , and ${\text{CO}}_{2}$ are Lewis acids, as they accept a pair of electrons from another species during the formation of a co-ordinate bond. Therefore, the Lewis base here is $\text{CO}$.