The correct option is C Upon cleaning my entire room, I found my ring on the floor.
While combining a
sentence with a preposition, it is often followed by a noun or a gerund.
When the given sentences are combined, a gerund/noun follows the preposition.
Option C uses the preposition 'upon' followed by the noun 'my entire room'. hence it is the correct option.
Option A does not make use of a preposition. It uses the conjunction 'and'. Hence A is incorrect.
Option B also does not use the preposition + noun/gerund combination. It uses the subordinating conjunction 'even though'. Hence B is incorrect.
Option D is incorrect because the answer lies in C.