Answer:
The correct answer is - D C2H4.
Explanation:
Saturated hydrocarbons are hydrocarbons with single covalent C-C bonds. They are known as alkanes. The general formula for these hydrocarbons is  CnH2n+2
Unsaturated hydrocarbons the hydrocarbons with double or triple covalent C-C bonds. They are known as alkenes and alkynes respectively. The general formula for these hydrocarbons is  CnH2n and CnHn-2
For the given options:
Option D: C2H4, is the simplest alkene with a double bond so it is an unsaturated hydrocarbon.
 
        
             
        
        
        
C is the correct answer (CaF2) (sorry dont have subscript)
Explanation: synthesis reaction forms a compound and calcium reacting with fluorine produces Calcium Fluoride (CaF2) chemical name
        
                    
             
        
        
        
Physical because it is still H2O
        
             
        
        
        
Answer:
C. Lithium 
Explanation: 
This image describes the trend in reactivity. Group 1 metals are always the most reactive so yeah :)