Best answer of those choices:
It's a little more complicated than just that -- let me explain. The Triple Alliance between Germany, Austria-Hungary, and Italy, was intended to be a defensive alliance. In Italy's view, both Germany and Austria-Hungary were acting as aggressors in starting the Great War (World War I). So it wasn't a "defensive" position they were in.
Additionally, Italy was seeing greater opportunity in siding with the Allies over against the Central Powers. Historically, Italy and Austria-Hungary had not been on the same side. Italy stood to gain some territorial concessions if it sided with the Allies against Austria-Hungary and won.
When the war began in 1914, Italy stayed neutral. By 1915, Italy declared war on Austria-Hungary and joined the Allies.