I would have chosen "reason" as the best available answer from those options also, and I think any test which gives "intention" as the sole correct synonym of "motive" without further context is simply badly constructed.
"Motive" most often means the purpose or reason for doing something, in my experience:
- The motive for this murder was to gain an inheritance.
- The political motive for this position was to appeal to the candidate's base voters.
"Motive" can also mean the intent with with the thing is done, and indeed the two meanings often overlap, as in my second example.
English words rarely have a single correct synonym, and often have multiple, often quite different senses, with different synonyms. There are no rules but usage for selecting which synonym is best, and without context one must guess at the intended sense, or the motive of the author.
(There are other senses of "motive" as well, but less common ones. "Motive power" for example. Ultimately a motive is that which moves something, usually in a metaphoric sense.)