We were very much (1)/ carefully in our approach (2)/ and hence we would (3)/ complete the complicated task. (4)/ No error (5)

Explanation

Carefully की जगह careful का प्रयोग होगा क्योंकि verb to be-is, are, am, was, were के बाद Adjective का प्रयोग होता है न कि adverb का । जैसे-She is careful.