It is both, a and b. The JAK-STAT-Pathway sits below the Erb-receptor and is therefore giving the signal from mutated Erb down the signalling cascade. Besides that a permanent active version of Jak2 has been found which also permanently activates this pathway. Both are contributing to cancer. See this two articles:

 - [Molecular Pathways: JAK/STAT Pathway: Mutations, Inhibitors, and
   Resistance][1]
 - [Mining for JAK-STAT mutations in cancer.][2]

The role of the MAP Kinase pathway in cancer is even more prominent. Mutations in different proteins of the cascade are know, like BRAF V6600E in melanoma, mutations in NRAS, KRAS, MEK and MEK2. Most of these are activating mutations which lead to a permanent activation of this signalling cascade and also a changed gene expression.
See this figure from the MAP Kinase paper mentioned below:

![enter image description here][3]

See these papers for more details:

 - [MAP kinase signalling pathways in cancer][4]
 - [Roles of the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR pathways in
   controlling growth and sensitivity to therapy-implications for cancer
   and aging][5]


  [1]: http://www.ncbi.nlm.nih.gov/pubmed/23406773
  [2]: http://www.ncbi.nlm.nih.gov/pubmed/18291658
  [3]: https://i.sstatic.net/XByms.gif
  [4]: http://www.nature.com/onc/journal/v26/n22/full/1210421a.html
  [5]: http://www.impactaging.com/papers/v3/n3/full/100296.html