Step 1) Look at the base
We can see that the base is a square, so most likely then this net is a pyramid.
Step 2) Look at the sides
We can see that there are 4 sides, and they are triangles.
By definition, a triangular pyramid only has 3 sides, which we know because it has 'triangle' in its name. But, the tetrahedron has 4 sides, which we know because it has the prefix 'tetra' in it, which means 4.
The correct answer is tetrahedron.
Hope this helps! :)