Bella said she has not received any gifts from her friend.
The error is in part A. Replace "has" with "had" to maintain past tense consistency. So, the correct sentence is: Bella said she had not received any gifts from her friend.

They told me that they are going to adopt another pet dog.
The error is in part C. "These days" implies present time, but the reporting verb ("told") indicates past tense, so it should be changed to "were going to adopt" to match the past tense. The correct sentence is: They told me that they were going to adopt another pet dog.

They said that they were planning a trip to Europe these days.
The error is in part D. "These days" indicates present time, but the main verb "were planning" is in the past tense. To maintain consistency, "these days" should be changed to "at that time" or a similar phrase indicating past time. So, the correct sentence is: They said that they were planning a trip to Europe at that time.

He said that he was working on an important project right now.
The error is in part D. The phrase "right now" indicates present time, but the reporting verb "said" is past tense. To maintain consistency, "right now" should be changed to "at that time" or a similar phrase indicating past time. The correct sentence is: He said that he was working on an important project at that time.

He told me that he had visited Paris with his family last year.
The sentence is correct as it is. There is no error in it.