Final answer:
The sentence that is written correctly is 'The bird that we saw in the yard is a robin.'
Step-by-step explanation:
The sentence that is written correctly is:
B. The bird that we saw in the yard is a robin.
In this sentence, the word 'that' is used as a restrictive relative pronoun to define the specific bird that the speaker saw in the yard. The use of 'which' in sentence A suggests that the bird in the yard is just one of many birds that the speaker saw, rather than a specific bird.