Explanation:

- In molecular MLx, M represents the central atom and x represents the number of atoms or groups attached to the central atom.
- The fact that the molecule is planar indicates that all the atoms or groups attached to the central atom are in the same plane.
- The valence shell of the central atom has 7 pairs of electrons, which means there are 14 electrons in total.
- To determine the value of x, we need to use the concept of electron pair geometry and molecular geometry.
- Electron pair geometry describes the arrangement of all the electron pairs (both bonding and non-bonding) around the central atom, while molecular geometry describes the arrangement of only the bonding electron pairs.
- In this case, the electron pair geometry is determined by the number of electron pairs around the central atom, which is 7.
- According to VSEPR theory, when there are 7 electron pairs around the central atom, the electron pair geometry is pentagonal bipyramidal.
- However, we are only interested in the bonding electron pairs for determining the molecular geometry.
- There are 5 bonding electron pairs around the central atom, which means the molecular geometry is trigonal bipyramidal.
- The value of x is equal to the number of atoms or groups attached to the central atom, which is 5 in this case.

Therefore, the correct answer is '5'.