The correct option is C (SiH3)3N+BF3
In (CH3)2O, O can donate its lone pair to the empty p orbital of BF3, whereas it is not possible in the case of (SiH3)2O and (SiH3)3N. This is because the lone pair of the central atom is already delocalised with Si via pπ−dπ backbonding. So, the lone pairs are not available for donation to the vacant p orbital of BF3. Hence, (SiH3)2O+BF3 and (SiH3)3N+BF3 can not form lewis acid base pair adduct.