The correct answer is "(Smith 179)".
The MLA in-text citation format comprises the author's last name and the page number of the author's work being referred to. These elements MUST NOT be separated by a comma or any other punctuation mark.
The option "(Smith 2015)" could be considered correct provided the author's work is 2015 pages long or more. It is most likely this number indicates the publication year of the author's work, rather than the page number. Therefore, this option is discarded.