a) to be most practical

b) most practical

c) to be the most practical

d) the most practical

e) as the most practical

Consider to be, consider as -> both are unidiomatic
Among (b) and (d)- i think b is the correct answer...

Experts can put some light

Correct idiom is "considered"; ignore A,C,E

we need noun form.. "the most practical" is best form; ignore B

must be [spoiler]{D}[/spoiler]

OK all are able to eliminate rest except B and D .out of these two D is answer !!
we need "the most ..."
the reason is we are pointing this particular solution from rest of the personal transport solutions.hence we need "the"
similar construction : he is one of the best teachers --->he is one out of the rest

rahul wrote :we need noun form..

i am not sure as why u have written this .well this sentence would have been good :Many consider two-wheeled vehicles practical
the reason is now that "of" construction is missing so we dont have to pinpoint using "the"

Thanks guys ! Now I find it easy (after knowing the answer )

A,C,E are unidiomatic
and with superlative (most) we should prefix "the" - of course another explanation for you using "the" could be that the "best" or "most" needs to be specifically pointed out.

That's it, D is the answer.

Thanks again