Knowledge of thermodynamics is required to design any device involving the interchange between heat and work, or the conversion of material to produce heat (combustion). Examples of practical thermodynamic devices: What is thermodynamics? The study of the relationship between work, heat, and energy
Answer:- 
Explanations:- Let's say the first molecule is A and the second molecule is B and the product molecule is C.
From given information, 2 molecules of A react with 3 molecules of B to produce 5 molecules of C.
We write the reactants on left side and the product on right side and there is an arrow between them from reactants towards the products.
So, the reaction could be represented as:

The catalyst (4) decreases the activation energy required for a reaction, by holding reactants in place
Pretty sure it's b but not an definitely