**Answer:**

**B2H6** is the electron deficient molecule.

- In order to determine which molecule is electron deficient, we need to consider the number of valence electrons in each molecule and compare it to the number of electrons required for a stable octet or duet.

- Boron (B) has an atomic number of 5, which means it has 5 electrons in its neutral state. Hydrogen (H) has an atomic number of 1, so it has 1 electron in its neutral state.

- In B2H6, we have 2 boron atoms and 6 hydrogen atoms.

- The total number of valence electrons in B2H6 is calculated by adding the valence electrons of each atom:
- 2 boron atoms x 3 valence electrons = 6 valence electrons
- 6 hydrogen atoms x 1 valence electron = 6 valence electrons
- Total = 6 + 6 = 12 valence electrons

- In order to determine if B2H6 is electron deficient, we need to compare the number of valence electrons to the number of electrons required for a stable octet or duet.

- For boron, it requires 8 electrons for a stable octet, and for hydrogen, it requires 2 electrons for a stable duet.

- Therefore, the total number of electrons required for a stable octet or duet in B2H6 is:
- 2 boron atoms x 8 electrons = 16 electrons
- 6 hydrogen atoms x 2 electrons = 12 electrons
- Total = 16 + 12 = 28 electrons

- Since B2H6 has only 12 valence electrons, it is electron deficient because it does not have enough electrons to achieve a stable octet or duet.

- On the other hand, molecules such as C2H6, SiH4, and PH3 have enough valence electrons to achieve a stable octet or duet, so they are not electron deficient.

- Therefore, the correct answer is option **B) B2H6**.