The correct answer is B. Keeping my mind on my writing is not easy when I stay at my grandmother's ocean-side cottage.
Step-by-step explanation:
In an argumentative text, the thesis refers to a statement that expresses the position of the author toward something and that is developed through evidence such as reasons, examples, descriptions, statistics, etc. during the text. This means the thesis statement, also called the main claim is the idea the author aims at explaining and proving. Considering this, in the text presented the thesis is "Keeping my mind on my writing is not easy when I stay at my grandmother's ocean-side cottage" because this statement shows the position of the author towards an issue that is writing on her/his grandmother cottage and also, this idea is the one the author explains and argues about by using a detailed description of the way she/he gets distracted while being there.