which one of the following sentences is correct? " The rains rarely drop" , " The rains rarely drops" , " The rains drop rarely" or " Rains drop...

These sentences are more about the use of rain and the
absence or addition of the word "the ". If you think of rain as a singular event
happening tonight, it is singular and requires a singular verb. For example, " rain
drops on my head if I stand outside" using the singular verb drops. If you consider "the
rains" with the addition of the word "the", you are using it as a plural such as a
series of storms passed through last night which requires a plural verb.  "The rains
drop on my head when I stay outside through several storms."    As for your sentences,
The rains rarely drop is correct because of the use of the word the, The rains rarely
drops is incorrect because of the singular verb with a plural subject, The rains drop
rarely is correct also.  I also think the verb drop is a problem because you can use it
as a noun counting each rain drop as it hits the windshield.  Try another verb to see if
that helps.  Use the verb "fall" instead which may make this easier to understand.
 English is a funny language, so don't give up. A dictionary can sometimes help you
determine if a word is singular or plural.