According to French Military officials, a uniquely agile robot
walking on four legs has proved useful to transport heavy equipment over long stretches of rough, mountainous terrain.
(A) walking on four legs has proved useful to transport
This option emphasizes the walking on four legs as the specific characteristic that's proven useful. This changes the intended meaning.
INCORRECT(B) walking on four legs has proved itself useful as a transport for
This option is similar to Option A and is too verbose. Also use of 'as a transport' changes the intended meaning.
INCORRECT(C) walking on four legs proves itself useful as transporting
This option is similar to Option A and B. Also use of present tense of 'prove' is wrong.
INCORRECT(D) that walks on four legs has proved useful in transporting
This option is right as it correctly uses the tense and relative pronoun 'that'. It conveys the intended meaning as well.
CORRECT(E) that walks on four legs proves itself useful to transport
This option is similar to Option C in terms of tense usage.
INCORRECTOption D is the right answer.