To determine the new period of oscillation when another mass $M$ is added to the spring system, we begin by recalling the formula for the period of a spring-mass system. The period $T$ of oscillation for a mass $M$ suspended from a spring is given by:
T = 2\pi \sqrt{\frac{M}{k}}
where $k$ is the spring constant.
When another mass $M$ is added, the total mass becomes $2M$. The new period of oscillation $T_{\text{new}}$ is:
T_{\text{new}} = 2\pi \sqrt{\frac{2M}{k}}
We can express the new period in terms of the original period $T$ by factoring out 2 from the expression:
T_{\text{new}} = 2\pi \sqrt{\frac{2M}{k}} = 2\pi \sqrt{2 \times \frac{M}{k}} = 2\pi \sqrt{2} \sqrt{\frac{M}{k}} = \sqrt{2} \times (2\pi \sqrt{\frac{M}{k}}) = \sqrt{2} \times T
Therefore, the period of oscillation when the mass is doubled is $ \sqrt{2} T $.
This means the correct answer is $ \sqrt{2} T $, making the correct option: $ \sqrt{2} T $.