Choose the correct sentence: I assured him of my sincere help., I assured him for my sincere help., I assured him of my sincerely help., I assure him to my sincere help.
Choose the correct sentence: I assured him of my sincere help., I assured him for my sincere help., I assured him of my sincerely help., I assure him to my sincere help.
Explanation
"Assure someone of something" is the correct preposition usage.
"Sincere" is the correct adjective; "sincerely" (adverb) would be incorrect here.