I can only guess that you are thinking of Walton's letters from Frankenstein.  I also think this should be a multiple choice question since there are hundreds of things that could be the reason he is writing his sister.  For example, the boat had to jeteson some weight and it had too much paper, so he decided to lighten the load by writing to his sister.  All right, a little much, but it is true.

The letters are a literary device.  They create a parallel person who is doing much the same thing as Victor, pursuing the "truth" in a dangerous area of knowledge (and in his case a physically dangerous place) whose experience is "real."  The letters make him seem more real, more believable.  This is the outer shell of the onion:  Walton|Victor|Creature.  Walton becomes someone Victor can tell his story to; the story is totally incredible, but telling it to a credible person makes it more acceptable to us.  When we get to the third layer, talking to a totally incredible creature, it seems more believable.

Almost anything else would be a reason that would "not be a reason" that he wrote to his sister.  Hope this helps.