Phosphorus has five valence electrons and hydrogen has one valance electron. what will be the lewis dot structure for the covalent compound phosphorus trihydride

The answer would be:

The first figure.

Here's more about the question:

Since phosphorus has 5 valence electrons, we know that it will need 3 hydrogen to share its electrons to complete the Octet (8 valence electrons)

When you draw the shared pairs we use bonds.                

Now we know that phosphorus is sharing 3 electrons to complete with the 3 hydrogens. But remember that it has 5 electrons, so that means it has 2 left over. These are what we call lone pairs. In drawing the LDS structure you have to show the lone pairs. They come in two, so draw two dots to represent them.