a -> Spamming can be used to obtain information about the individuals. Those emails can contain links that try to reveal your sensitive information. They can also contain viruses directly like in the some attached folder. Another usage of spamming is just to advertise some products.
c -> Spam emails are not something you requested for. They are just sent to email addresses in huge amounts.
d -> As mentioned earlier, they can be used to obtain personal information like password, credit card information.
