Correct Answer: Mature

Solution : The correct choice is the third option .

The term young refers to the early stage of life, or the opposite of old age. In this context, the opposite of "young" is mature , as maturity generally implies full development or the state of being fully grown and adult.

The meanings of other options are as follows:

• Limited: It refers to something that is restricted in size, extent, quantity, or scope
• Silly: It refers to a lack of good sense or judgement.
• Gravity: It typically refers to the force that attracts objects towards the centre of the Earth.

Therefore, the correct answer is mature.