Force not acting in an enzyme substrate complex
This question is a controversial one.
The substrate bind to the active site of the enzyme by noncovalent interactions like hydrogen bond, ionic (electrostatic) bond and hydrophobic bonds.
There is confusion between noncovalent van der Waals forces and covalent bond. Some guides have given option `c' as the answer, which is incorrect.

"Enzymes bind to the substrate by a combination of hydrogen and ionic (electrostatic) bonds, as well as by hydrophobic and van der Waals interactions". 
"Most enzyme-substrate combinations are mostly due to noncovalent hydrogen, hydrophobic, ionic (electrostatic) and Van der Waals forces".       
"There is a precise substrate interaction that occurs at the active site of enzyme stabilized by numerous weak interactions (hydrogen bonds, electrostatic interactions, hydrophobic contacts, and Van der Walls forces)".

Many other references have mentioned the same.

Thus Van der Walls forces are also involved in enzyme substrate reaction.
Now we are left with covalent bonds. Covalent bonds are also involved in some enzymatic reactions called covalent catalysis. In which there is the formation of covalent bond between enzyme and substrate. The modified enzyme then becomes a reactant. For example, the serine residue at the active site of chymotrypsin forms a covalent link with the substrate.
So, all the four forces given in the options are involved in the enzyme-substrate interaction. However, amongst the given options, I will go for covalent bonds because most of the enzyme-substrate interactions involve weak non-covalent interactions, whereas covalent catalysis mechanism is found only in few enzymes like proteases (chymotrypsin and trypsin) and few others.

The best option, in a given scenario, the answer is a covalent bond.
Electrostatic
Covalent
Van der waals
Hydrogen