Select the most appropriate option to fill in the blank. Meera and Rahul live…

Concept: In an English noun phrase the slot between a determiner and its noun — a ___ house — is filled by a modifier, and the word class of that modifier decides what it can express. When the sentence has to state a QUALITY of the noun, that modifier must be an adjective: an adverb cannot occupy the slot at all, and a noun placed there only builds a compound that names a type or field instead of describing the noun.

Application: The blank has to state a quality of the house, and the quality intended here is 'having plenty of room'. The noun 'space' takes the adjective-forming suffix -ous to give 'spacious', which carries exactly that sense, so the sentence reads: Meera and Rahul live in a spacious house.

Contrast the four forms built on the same root:

Cross-check: put each form back into the frame 'a ___ house'. 'a spaciously house' is ungrammatical, 'a space house' would name a kind of house rather than describe it, 'a spacy house' would call the building dazed, and 'a spacious house' states exactly the intended quality of plenty of room.