ANS:A - solid solution strengthening.

The primary strengthening mechanism in 70:30 brass is precipitation hardening. Brass is an alloy primarily composed of copper and zinc, with varying compositions depending on the specific type of brass. In the case of 70:30 brass, it contains approximately 70% copper and 30% zinc. Precipitation hardening involves the formation of fine precipitates within the metal matrix, which hinder the movement of dislocations and thus increase the material's strength. In 70:30 brass, the precipitation of a phase rich in zinc, typically referred to as the η (eta) phase, is the main mechanism responsible for strengthening the alloy.