Let $X$ be continuous Markov process adapted to a filtration generated by Brownian motion $B$. Does there exist a function $f$ such that $X_t = f(t,B_t)$?
My guess is that it should have such representation. However, I am not able to produce any proof.
No. Try $X_t=S_t-B_t$, where $S_t=\sup\limits_{0\leqslant s\leqslant t}B_s$.
Another class of examples: $X_t=\displaystyle\int_0^tg(s)\,\mathrm dB_s$ with $g$ not identically $0$, not identically $1$, and not identically $-1$.