How does Paragraph 5 develop the idea that playing an instrument or singing can help develop the brain? It provides details of a research study on ways to improve executive function skills in young children.

It contrasts the effects of singing and the effects of learning to play an instrument, both of which affect different regions of the brain.

Music and the Brain
It suggests that learning to play an instrument may be a useful intervention for people with brain injuries or brain diseases such as Alzheimer's.

It describes specific areas of the brain that benefit from learning to sing or play an instrument.

Paragraph 5 in that question surely develops the idea that playing an instrument or singing can help develop the brain. It describes specific areas of the brain that benefit from learning to sing or play an instrument.

Explanation:

To identify the idea of a paragraph, we have to read the whole paragraph and get the important information in it. The whole paragraph 5 above mostly tells about the parts of the brain which include the process of interpreting sensory information, like playing instruments or singing. It describes one by one, parts of the brain that are affected by music.

For example, both rights and left sides of the brain are activated when we play or listen to music. These parts of the brain are being stimulated by music where this condition will increase the efficiency in processing information. The center of the brain which takes responsibility to figure out what must get done and how to do it is also developed by music. Researchers even find out that the center of the brain is more developed in musically trained students than in those who are not well trained.  

Thus, it can be said that paragraph 5 surely develops the idea that playing an instrument or singing can help develop the brain by describing specific areas of the brain that benefit from learning to sing or play an instrument.

in our power to make a world happy - to teach mankind the art of being so - to a character hitherto unknown - and to have, as it were, a new creation intrusted to our hands, are honors that command reflection, and can neither be too highly estimated,