The number of species having 3c-2e- bond among the given options is 3.

Explanation:
To determine the number of species with a 3c-2e- bond, we need to analyze the electron count and bonding situation for each compound.

1. Al2(CH3)6:
In this compound, we have two aluminum atoms (Al) and six methyl groups (CH3). Each methyl group contributes one carbon atom and three hydrogen atoms. Aluminum is in the +3 oxidation state, so it has 3 valence electrons. The carbon atom in the methyl group has 4 valence electrons, and each hydrogen atom has 1 valence electron. Thus, the total electron count is:
2(3) + 6(4) + 6(1) = 36 electrons
The compound has a 3c-2e- bond because each aluminum atom is bonded to three carbon atoms via covalent bonds, and each carbon atom shares two electrons with the aluminum atom. Therefore, Al2(CH3)6 has a 3c-2e- bond.

2. AI2(CH3)4CI2:
In this compound, we have two aluminum atoms (Al), four methyl groups (CH3), and two chlorine atoms (Cl). Each chlorine atom contributes 7 valence electrons. The electron count is:
2(3) + 4(4) + 2(7) = 34 electrons
Although there are 3c-2e- bonds between aluminum and carbon atoms in the methyl groups, the presence of chlorine atoms affects the overall electron count and bonding situation. The electrons from the chlorine atoms form additional bonds, which do not follow the 3c-2e- bonding pattern. Thus, AI2(CH3)4CI2 does not have a 3c-2e- bond.

3. (MgH2)2:
In this compound, we have two magnesium atoms (Mg) and four hydrogen atoms (H). Each magnesium atom contributes 2 valence electrons, and each hydrogen atom has 1 valence electron. The electron count is:
2(2) + 4(1) = 10 electrons
There are no carbon atoms present in this compound, so it does not have a 3c-2e- bond.

4. AI(BH4)3:
In this compound, we have one aluminum atom (Al) and three borohydride ions (BH4-). Each borohydride ion contributes 4 valence electrons. The electron count is:
1(3) + 3(4) = 15 electrons
Although there are 3c-2e- bonds between aluminum and carbon atoms in the borohydride ions, the overall electron count and bonding situation are affected by the presence of the boron atoms. The boron atoms form additional bonds, which do not follow the 3c-2e- bonding pattern. Therefore, AI(BH4)3 does not have a 3c-2e- bond.

5. BeCI2 (in solid):
In the solid state, BeCI2 does not exhibit a 3c-2e- bond. It forms a regular ionic crystal lattice, where beryllium (Be) gives up its two valence electrons to chlorine atoms, resulting in the formation of Be2+ and 2Cl