The sentence is not correct either. That does not belong in the sentence
The question should be:
"The number of birds observed in the pond is a very few/very small. "
To solve these types of questions, take away the unnecessary clauses.
The number is a very few.
The number is very small.
What is needed is an adjective to modify the word number. So the correct answer is: "The number of birds observed in the pond is very small. "
Student or Learner