OPTION A) Take language a^nb^n now make single tape turing machine of it you will find that Time to accept this language is o(n^2)=T(n).
For the same language with multi-tape, store equal numbers of a and b in different tape here machine accept the language in o(n) time.
One must note that having a multi-ptape does not increase the power of TM,it can reduce the time to accept some language like in above case