6395. In spite of their efforts, the team of scientists could not make much headway to solve the problem.

✅ The correct answer is option A.
progress : develop towards an improved or more advanced condition.

thinking : the process of considering or reasoning about something.

efforts : a vigorous or determined attempt.

start : begin or be reckoned from a particular point in time or space; come into being.

headway : forward movement or progress, especially when this is slow or difficult.

Synonym of progress is headway