The answer is b, as the mutation constantly activates the RAS protein. RAS is part of the MAP-Kinase pathway, constant signaling of it permanently activates this pathway and leads to changes in gene expression. See the image below:

![enter image description here][1]

If you are interested in more details, have a look at these papers:

 - [RAS oncogenes: weaving a tumorigenic web][2]
 - [RAS mutations and oncogenesis: not all RAS mutations are created
   equally][3]


  [1]: https://i.sstatic.net/d2Kms.png
  [2]: http://www.nature.com/nrc/journal/v11/n11/full/nrc3106.html
  [3]: http://journal.frontiersin.org/Journal/10.3389/fgene.2011.00100/full