Which sentence uses the word them as an indirect object?

a. the students are at the launch site with them.
b. leo sent them a postcard from nasa.
c. space exploration is a fascinating subject to them.
d. the tour guide took them around nasa.

I am pretty sure that the sentence which uses the word ''them'' as an indirect object is the last one : d.The tour guide took them around NASA.  As you know,  direct object stands near the verb in order to point on the thing that being acted upon. Here is two main points : took-V, them-indirect object, as a recipient of the direct one.

Do hope you will find it useful.