Step 1: Identify what the question is testing.
This question tests the choice between "whoever" and "whomever", which depends on the grammatical role the word plays inside its own clause.
Step 2: Break the sentence into clauses.
The sentence has two clauses: "A wise and experienced manager will assign a job to..." and "...is best qualified". The word in question sits at the junction of these two clauses, so we must look at its role inside the second clause, "[who] is best qualified".
Step 3: Determine the correct case.
In the clause "is best qualified", the pronoun is the subject of the verb "is". A subject must take the nominative (subject) form, which is "whoever", not the objective form "whomever" (or the incorrect two-word spelling "whom ever"). It does not matter that the whole clause functions as the object of the preposition "to"; what matters is the pronoun's job within its own clause.
Step 4: Correct the sentence.
The sentence should read "A wise and experienced manager will assign a job to whoever is best qualified."
Step 5: Locate the error.
This faulty pronoun appears in segment (d), "a job to whom ever is best qualified." Segments (a), (b) and (c) are correct, so the answer is option (d).