The 'Probable Answers' for Right Form of Verbs states that the correct form for blank (a) is 'has been raining'.
Class 6 › English Grammar and Composition › Chapter 11: Answer Key, Sample Question and Guidelines › Topic: Grammar Section (Parts of Speech, Gap Filling, Sentence Making, Verb Forms, Sentence Transformation, Punctuation)
Aligned to the NCTB national curriculum.