Hi anapam, and welcome to English Forums.
India IS - the main subject is India and the verb is "is," which matches the singular subject.
"one of the foremost countries" - this part takes a plural noun. It's not the subject of the sentence. In the following expamples, the subect and simple verb are bolded. You see the same "One of the___ plural-nouns" again.
He is one of the friendliest cats I've ever met.
She is one of the best teachers in the school.