Gottlob Frege, who was a philosopher and mathematician, undertook projects both investigating standard mathematical topics such as geometry and analysis, and the logical foundation of mathematics in an abstract sense.
A. Gottlob Frege, who was a philosopher and mathematician, undertook projects both investigating
B. Gottlob Frege, the philosopher and mathematician, undertook projects that were investigating both
C. Philosopher and mathematician Gottlob Frege, who undertook projects investigating both
D. Philosopher and mathematician Gottlob Frege undertook projects investigating both
E. Philosopher and mathematician Gottlob Frege undertook projects that were investigating both
i) First of all the structure should be investigating both for parallelism. - A out
ii) Secondly projects that were investigating seems to suggest as if projects were actually investigating instead of that the projects were meant to be investigating - B and E out
C doesn't have a verb thus a fragement - Out
Hence D - Correct