Answer:
simple
Explanation:
Simple sentence has one subject and one verb. e.g. I am going to visit a doctor on Saturday.
Compound sentence includes two separate sentences which are connected with a conjunction.  e.g. I enjoy watching drama movies, but my sister hates them. 
Complex sentence is made of two clauses, one dependent clause and one independent clause.  e.g. because I was in a hurry, I had to travel by plane. 
<u>The sentence ‘Halley's comet should return in the year 2061’, is a simple sentence.  </u>
 
        
             
        
        
        
Yes, I believe so. The two sentences in the paragraph use similar/same-ish sentence structure.