The correct option is A junk mails
In the given sentence, the phrase, 'a lot of' refers to a plural number of something.
Hence, the correct option must also be plural.
Options A and C: 'Junk mails' describes the number of junk e-mails received in the junk folder.
'Junk mail' is a singular noun, it refers to a particular type of mails: they are not significant, like promotional emails and offers on products and services. They are called 'junk'. The option does not describe the mails in terms of quantity 'a lot of'. Hence option A is correct and option C is incorrect.
Options B and D: 'Junks' is incorrect in both cases; the word 'junk' is an adjective and not a noun, it modifies the noun 'mail' or 'mails'.
Adjectives cannot be plural. Hence B and D are both wrong.