He is very weak. He cannot walk. (Choose the option with the correct combination of the above two sentences by using an infinitive)

He is too weak to walk.

He is very weak and cannot walk.

He is weak and therefore cannot walk.

None of these

The phrase "too weak to walk" correctly uses the infinitive ("to walk") to combine the two sentences.

The structure "too + adjective + to + verb" expresses an inability due to the adjective (weakness in this case).