Concept:The blank needs a connector that contrasts a long record of success with the challenges that accompanied it.Explanation:The clause "didn't come out without challenges" means the success was not free of difficulties.The blank sits between two commas, so a parenthetical transition word fits naturally."However" is a conjunctive adverb used to introduce a contrast or qualification.The full sentence becomes: "His many years of success in legal practice, however, didn't come out without challenges."
"But" cannot be simply inserted between commas; it normally joins two complete clauses."In spite of all" is a phrase, not a single connector, and it sounds awkward in this position."Indeed" would emphasise the statement, but it does not provide the needed contrast.Therefore, "However" is the most appropriate option.Answer:D. However