Dear Fred:

Here is the passage in question:

Now when he rose early on the first day of the week, he appeared first to Mary Magdalene, from whom he had cast out seven demons. She went out and told those who had been with him, as they mourned and wept. But when they heard that he was alive and had been seen by her, they would not believe it.  

After this he appeared in another form to two of them, as they were walking into the country. And they went back and told the rest, but they did not believe them. 

Afterward he appeared to the eleven themselves as they sat at table; and he upbraided them for their unbelief and hardness of heart, because they had not believed those who saw him after he had risen. And he said to them, "Go into all the world and preach the gospel to the whole creation. He who believes and is baptized will be saved; but he who does not believe will be condemned. And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; they will pick up serpents, and if they drink any deadly thing, it will not hurt them; they will lay their hands on the sick, and they will recover." 

So then the Lord Jesus, after he had spoken to them, was taken up into heaven, and sat down at the right hand of God. And they went forth and preached everywhere, while the Lord worked with them and confirmed the message by the signs that attended it. Amen.

Many biblical scholars in the past century believe that Mark 16:9-20 is a later addition to the Gospel. The reason they think that is that the passage is not found in some of the older extant manuscripts and early witnesses. They also claim there are stylistic differences in this passages that imply they were not part of the original. Other scholars believe the passage are part of the original.

None of this matters.

God gave the Pope and Magisterium the charism and authority to declare what is and what is not authentic Scripture.  Thus, whether or not Mark 16:9-20 is a later addition is moot as the Church, in its authority give to her by God, as declared that theses passages are a genuine part of Mark's Gospel.

Here are the Catholic footnotes for this passage of Mark 16:9-20 from the RSV Catholic 2nd Edition:

Other ancient authorities omit versus 9-20. Some ancient authorities conclude Mark instead with the following: But they reported briefly to Peter and those with him all that they had been told. And after this, Jesus himself sent out by means of them, from East to West, the sacred and the imperishable proclamation of eternal salvation.

This passage is regarded as inspired and canonical Scripture even if not written by Mark. As it is missing from some important manuscripts, it is possible that Mark did not write it. On the other hand, he would hardly have left his Gospel unfinished and verse 8. Many think that the original ending was lost at a very early date and that this ending was composed at the end of the apostolic period to take its place.

The bottomline: the Church has declared this passage as inspired and canonical Scripture regardless of who wrote it or when it was written.

God Bless,
Bro. Ignatius Mary