Apostrophes

Which one of the following sentences illustrates correct uses of apostrophes?

a. When writing out those POs, please be sure to dot your i's and cross your t's.

b. Mr. and mrs. Winston P. Oglethorp III cordially invite you to an hors d' ouvre reception at five o clock in the evening on the tenth of June.

c. The old man recollected, "Youngster, 29' was a miserable.

d. That hospital's RNs uniforms are dark green.

I chose A is this correct. I am a tutor and sometimes this is confusing for me. Thanks TK