In most modern businesses, there is now one manager for

I think A is the cleanest here. The accepted format is for every X there is a Y.

I think managers have employees and not the reverse.

E is unidiomatic because it uses "as many than". B and D change the meaning of the sentence. In addition in D there is not subject/verb agreement. Only A and C remain. Between A and C, A is better because it keeps the meaning. In C the meaning is reversed.

A is the best.

A looks good. Good idiom usage.

B- sentenc structure awkwar, was is incorrect
C-was is incorrect
D - awkward sentence structure
E - wrong idiom

OA is A.

OE is really detailed:

Spot the Concept Being Tested:
Idioms: Recognize your idiomatic triggers and their rules.
Verbs: Look for modifiers inserted in to the middle of the sentence, two or more distinct time frames in the sentence, the have + verb・form in the underlined part, or quantity words.

Decide on a Fix:
The Idiom in question is as ・as・and the idiom is used correctly here.
The rule for Verbs is that the verb must agree with its subject and must make time frames as clear as possible.
In this case we have to take a close look at the verb there・ at the end of the sentence. The verb described the number of managers in the past. Since we know that the number in the present is 1, then the past must have been greater than that. Therefore, the verb must be plural.

POE:
Take the most solid of these rules and eliminate answer choices.
Idioms are always the best choice if they are present.
Only E uses the idiom incorrectly.
Eliminate E.

Chunk and Compare:
Compare A to C.
The first difference is changing the order of the ・0 employees・and the 双ne manager・
Since neither version violates any of our rules, move on and look for another difference.
The next difference is the there・in Choice A and the was・ in Choice C.
Since the subject is now ・0 employees・the singular verb in C is incorrect.
[Note: If you cannot find a difference that matters, ALWAYS be biased toward the original answer.]
Eliminate C.

Compare B to D.
The first difference would be have・in Choice D versus has・in Choice B.
The difference between has and have is one of quantity.
Since we are speaking about the ・0 employees・we must use the plural.
Eliminate B.
Compare A to D.
Put the order of the subject aside, because we already decided that this did not seem to make a difference.

The next difference is or・in Choice A versus the verb was・ in Choice D.
When have is used alone it connotes possession.
The sentence is expressing a ratio or relationship there is no possession.
Therefore have・cannot be correct.
Eliminate D.

Reread Sentence With Your Choice and Choose an Answer.
Choose A.