Transform the following sentence to avoid using 'too':
He was too tired to run. [Intermediate]
Let's understand the question first. The sentence "He was too tired to run" is a simple sentence containing one finite verb 'was'. Although 'was' is an auxiliary verb, it is considered a main verb as it is the single verb in the sentence.
Now, during transformation of a simple sentence to a complex sentence, we replace the 'too...to' structure with 'so...that' structure, where 'so that' is the subordinating conjunction.
One thing needs to be kept in mind during such transformation and that is when the 'too...to' structure is positive, the 'so that' structure becomes negative and vice versa.
Hence, the correct answer would be "He was so tired that he could not run". Thus, option C becomes correct and the other options A, B and D become incorrect because they do not comply to the correct grammatical orientation.