None of the choices are good, and neither is the sentence. The sentence should read "the transportation system must be optimized to minimize cost. The words "for people to move" are superfluous. What else could a transportation system possibly do? "Cost" is the thing being optimized so it ought to be the direct object of the verb "optimize". All the choices are wrong. The correct answer is "minimize" but given that that choice is not offered, I would pick "idealize" as the next best answer. It is acceptable (given the choices) because minimization is the ideal they wish to achieve.

Looking at question 17 I can see this is a Chinglish test so who knows what the right answers are.

One way to view it is an enhanced version or an optimal solution improved based on a previous or less desirable implementation or performance. In contrast, an ideal scenario is merely a thought comparing the perfect mental perception and reality; therefore, we add embellishment as a patch-up when we idealize life situations. Compared with a more active role in the formers, the latter takes on a more passive attitude, as in waiting for something ideal to happen in the foreseeable future.

I suspect that the compilers of these tests think that ‘idealised’ is the correct answer. As others here have noted, this word is not used in this way in standard English ; however, this may be one of those cases in which you have to see it in the same way as them*, so go for D. Incidentally the word ‘optimise’ is only used for formal writing of this kind, and you are never likely to use it in everyday situations. * to ‘put yourself in their shoes’ as we say

Optimize means to make as perfect or effective as possible. None of the four choices has the same meaning but enhanced is the closest fit as it implies improving something (making it closer to perfect). Idealize does not mean to make ideal, it means to hold or consider someone/something to be ideal, so it's quite different from optimize. Realized and optional are both different in meaning. However, I'm a US English speaker so other English speakers may have differing points of view.