It D machinery. It says it on the bottom of the problem but for further explanation, a mail room can be found in some but it’s more in offices than factories, a barn is obviously wrong, and cafeteria is only in some not all but all factories have machinery used to make the item they produce.
I'm pretty sure its 100 because if she gave each of her 19 students 5 invitations that equals 95. (19×5=95) And She had 5 leftover equals 100. (95+5=100)