Each of the following sentences contains a highlighted section that may be grammatically incorrect. From the options provided, select the one that best corrects the highlighted portion. If the sentence is already correct, select "No improvement needed".

A devious manager, the dishonest supervisor continued to deny mistreat even though there were multiple complaints.

The correct option is to deny mistreating even though.

Let's analyze the grammatical principles needed to correct the highlighted portion of the sentence:

1. Infinitive Verb Construction:
The main verb "continued" is followed by an infinitive phrase. The standard infinitive form is "to" followed by the base form of the verb, which is "to deny". Phrases like "to denying" are grammatically incorrect in this context.

2. Gerund Object of the Verb "Deny":
The verb "deny" takes a gerund (a verb ending in -ing acting as a noun) as its direct object to indicate an action being denied. Therefore, "mistreat" (base verb form) must be changed to the gerund form "mistreating". Saying "to deny mistreat" is grammatically incorrect.

3. Subordinating Conjunction Choice:
The phrase "even though" is a standard subordinating conjunction used to introduce a clause of concession. While "even although" is redundant and non-standard, "even though" correctly connects the main clause with the subordinate clause ("there were multiple complaints").

Combining these corrections yields: to deny mistreating even though.

Thus, the complete correct sentence reads:
"A devious manager, the dishonest supervisor continued to deny mistreating even though there were multiple complaints."