Sentence ending のを(だ)

Question

I do not understand the meaning of のを:

a. 彼は家を買った。 しかも大きい庭付きのを(だ)。 (だ is indicated as optional)

Is it different from:

b. 彼は家を買った。 しかも大きい庭付きだ。 (I'm not sure if this is grammatical)

c. 彼は家を買った。 しかも大きい庭付きのだ。

What does it mean when the copula takes on a direct object? I have so far only encountered AはBだ,　AがBだ, Bだ but I have not seen を directly preceding the copula before.

Answer 1

It is a (pseudo) cleft sentence with the noun phrase and the topic ellided. I thought there was a variety among native speakers who accept を and who don't.

しかも、かれは大きい庭付きの家を買った
'In fact, he bought a house that has a large garden.' (Original sentence)

しかも、彼が買ったのは大きい庭付きの家(を)だ
'In fact, what he bought was a house that has a large garden.' ((Pseudo) cleft)

しかも、彼が買ったのは大きい庭付きの(を)だ
'In fact, what he bought was one that has a large garden.' (Ellipsis of a noun phrase)

しかも、大きい庭付きの(を)だ
'In fact, (what he bought was) one that has a large garden.' (Ellipsis of the topic)

It is different from the other sentence, in which simply 家 is ellided:

しかも大きい庭付きだ　
'In fact, (it) has a large garden.'

Answer 2

In a., -no is the genitive case marker that connects two noun phrases, and -wo is the accusative case marking the direct object of the verb. The problem that you are likely having is in understanding the ellipse coming from the first sentence. You should interpret it as follows: 彼は家を買った。しかも大きい庭付きの（家）を（買った）。 "He bought a house. In fact, a big one with a garden."