The sentence intends to compare the space of the said house with the previous one.
For comparison, the comparative degree adjective should be used. So, 'more spacious' would fit the blank.
The correct sentence is: The house they bought was more spacious than their previous house.
Hence, option (d) is the answer.
Note: · The Positive Degree is used to denote the mere existence of some quality of what we speak about. It is used when no comparison is made.
· The Comparative degree denotes a higher degree of quality than the positive and is used when two things are compared.
· The Superlative degree indicates the highest degree of quality and is used when more than two things (or sets of things) are compared.
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