The singular noun in this sentence would be the word "Sand" because it's talking about the "Hot sand".
He walked across the hot "sand", hot is used to describe the word sand. 
Thus your answer.... B. sand. 
Hope this helps. :)
        
                    
             
        
        
        
<span>The connotative meaning of sky is “limitless." The sky is limitless for a bird who has freedom.
The connotative meaning of narrow is “limited.” The narrow cage is limited/restrictive for a bird who's caged.
The connotative meaning of dreams is “hopes and desires.” Even though caged, the bird still hopes and dreams for his freedom.
The connotative meaning of shadow is “who a person once was.” The shadow refers to what the bird once was, free and unconfined.</span>
        
                    
             
        
        
        
¡Hola! 
1. El medio ambiente mejora
2. La contaminación ambiental se reduce . 
3.                                
Espero que te sirva