Social scientist: Since the body of thought known as Marxism claims to

Let's try to decode the passage:
Social scientist:
Since the body of thought known as Marxism claims to describe rigorously an inexorable historical movement toward the socialization of the means of production, it should be regarded as a scientific theory.
Simple Language --> Becoz marxism described the historical movement of how means of production were impacted by socialization, it is a scientific theory.
Thus, certain interpreters, in taking Marxism as a political program aimed at radically transforming society, have misconstrued it.
Simple Language --> interpreters are wrong as they think of Marxism as a political movement

Now let's look at the answer choices, we need to find the assumption that can link the two parts:

(A) The description that Marxism gives of certain historical phenomena in the economic sphere is as rigorous as it is claimed to be.
Just because the description is rigorous it doesn't lead to the conclusion. Incorrect

(B) The aims of science are incompatible with the aims of those who would seek to transform society by political means.
This is too broad as it compares all of science with all of politics. Incorrect

(C) Only bodies of thought consisting purely of rigorous description are scientific theories.
Only tells us what scientific theories are, not enough to link the two statements.

(D) Scientific theories cannot be correctly interpreted to be, or to imply, political programs.
Now, this helps us understand why "Scientific Theories" cannot be "Political programs" and hence connects the conclusion on the 2nd statement with the first one.

(E) The means of production will inevitably become socialized regardless of any political programs designed to make that occur.
Always would have become socialised irrespective of political programs or not. The interpreters still might come to say that the intent was political. Incorrect.

Hence D is the correct answer.