Define a public good and then describe which of the following are public goods and explain why and what public

a. medical research on cosmetic surgery
b practice of open heart surgery
c orthodontic work
d aids education
which are public good and explain why and what public polices are justified if the good is a public one.

For define of public good, you can define itself by use the characteristic of public good:
1. Nonrivalry: If you consume it does not diminish the amount of it for the others.
2. Nonexcludability: If someone consume it, the others can't be excluded to consume too.
Any public good have degree 100% for two characteristics as national defence, safety feeling from jail (pure public good). The private good, as shoes, shirts etc., have 0% degree for above characteristics (pure non public good). Fish in the lake have 0% nonrivalry and 100% nonexcludability (non pure public good), and football team 100% nonrivalry and 0% nonexcludability (non pure public good). So you can find the examples of 50% nonrivalry and 50% nonexcluabilty.
Public policies are justified to it, because the government concern to give the best service for public. For example, garden park for health of people is the main policy of govt.

Do your own homework.

I am the first one!!

A public good is a good where the use by one person does not prevent someone else from using it. In economics, It does not mean good for people .
a.
The market will under supply public goods because once created they are no longer scarce so can;t be sold. The government issues patents and copyrights to restrict use of knowledge developed by people so they can sell it, or the government may pay for creation of public goods directly from taxes.