Arachno type structure refers to a type of cluster geometry where the metal atoms are arranged in a three-dimensional shape resembling a spider or spider web. The correct answer is option B, [Os3(CO)12], which has an arachno type structure.

Explanation:

1. Cluster Compounds:
Cluster compounds are molecules consisting of metal atoms or ions, typically in a small cluster of two to several atoms, surrounded by other atoms or ligands.

2. Arachno Type Structure:
Arachno type structure is a type of cluster geometry where the metal atoms are arranged in a three-dimensional shape resembling a spider or spider web. This structure is characterized by the presence of three types of vertices, which are called arachno, meta and ortho vertices.

3. Option A [Os5(CO)16]:
This cluster compound contains five osmium atoms and sixteen carbon monoxide ligands. It has a trigonal bipyramidal structure, not arachno type.

4. Option B [Os3(CO)12]:
This cluster compound contains three osmium atoms and twelve carbon monoxide ligands. It has an arachno type structure, which is characterized by the presence of three types of vertices: arachno, meta, and ortho vertices.

5. Option C [Ir4(CO)12]:
This cluster compound contains four iridium atoms and twelve carbon monoxide ligands. It has a tetrahedral structure, not arachno type.

6. Option D [Ph6(CO)16]:
This cluster compound contains six phenyl groups and sixteen carbon monoxide ligands. It has a cuboctahedral structure, not arachno type.

Conclusion:
The correct answer is option B, [Os3(CO)12], which has an arachno type structure.