What protocol does a web browser normally use? at which osi layer does this protocol work?

The answer is HTTP and it works over the application layer. The hypertext transfer protocol or as called as HTTP is an application protocol for distributed, collaborative and hypermedia information systems in which this is the foundation of the data communication for the world wide web or as called as www. 

For question 1, i'm a bit stuck between serif fonts or large enough type to read. serif fonts are good fonts to use because their easy to read. but you also need your type be large enough to read. if i had to say what you need to consider most important, i would say d is the answer.  for question 3, i believe the answer is d. it seems most appropriate to do that. on every page or every other page would be too much, and only on the contact page just seems inappropriate.  for question 4, it's probably a. by "color scheme" it doesn't mean just one color, but a set of colors. colors that are complimentary to each other. having too many is too distracting so it's best to limit it as much as possible, but not to the point where your website looks dull.hope this !