As there were not enough seats to ........... so many people at the venue of the address, they had to put up a big tent outside.

The sentence is talking about the instance where the number of seats were insufficient. While entertain and welcome both can be used to talk about the number of seats, the sentences talks from a more managerial perspective. 

Ascertain means to confirm something, which does not it the context here. 

Accommodate would be the given context where it would mean space for lodging or making room for the people at the venue. 

Therefore, Option D is the correct answer.