The condition is that $d$ should be optimal. Formally: A prefix-free machine is a partial computable function $M:2^{<\omega} \rightarrow 2^{<\omega}$ such that $M(\sigma)$ halts implies $M(\tau)$ does not halt for any $\tau$ comparable with $\sigma$. For such a machine $M$, Let us define $K_M(\sigma)$ to be $\min \{|\tau| : M(\tau) = \sigma\}$
A prefix-free machine $U$ is optimal if for any prefix-free machine $M$, there is a constant $c$ such that for every $\sigma$ we have $K_U(\sigma) \leq K_M(\sigma) + c$.
Then $$\Omega = \sum_{U(\sigma) \text{ halts}} 2^{-|\sigma|}$$ is a normal number in every bases, and much more: It is Martin-Löf random, that is, there is a constant $c$ such that for every prefix $\sigma$ of $\Omega$ (seen as an element of $2^{\omega}$), we have $K_U(\sigma) \geq |\sigma| - c$. It means that $U$ cannot compress the prefixes of $\Omega$. By optimality of $U$ it means that no prefix-free algorithm can compress prefixes of $\Omega$.
The key point of the demonstration lies in the fact that from the $n$ first bits of $\Omega$, one can uniformly decide if $U$ halts or not on strings of length smaller than $n$: Run $U$ on every strings in parallel and approximate $\Omega$ from below by adding $2^{-|\sigma|}$ each time $U$ halts on $\sigma$. At some point our approximation will match the $n$ first bits of $\Omega$ and we then know if $U$ halts or not on strings of length smaller than $n$.
Suppose now for contradiction that for every $c$ there exists $n$ such that $K_U(\Omega \upharpoonright n) < n - c$. We can build the prefix-free machine $M$ which on $\sigma$ does the following: $M$ runs $U(\sigma)$ until it outputs some $\tau$ (the interesting cases being when $U(\sigma) = \Omega \upharpoonright n$ for some $n$). Then $M$ tries to decide what are the strings $\rho$ of length smaller than $|\tau|$ such that $U(\rho)$ halts, by the algorithm described above. If $M$ succeeds deciding it (or think it succeeds), then it output a string different from $U(\rho)$ for any such string $\rho$.
Note that as long as $U(\sigma) = \Omega \upharpoonright n$, we must then have $K_U(M(\sigma)) \geq n$, by design. But up to some constant we also have $K_U(M(\sigma)) \leq K_M(M(\sigma)) \leq |\sigma|$ (by optimality of $U$ and definition of $K_M$). In particular $|\sigma| \geq n$ up to some constant. Note that the constant depends on the machine $M$ only and not on $\sigma$ or $n$. Now by hypothesis, the difference between $|\sigma|$ and $n$ for strings $\sigma$ such that $U(\sigma) = \Omega \upharpoonright n$ can be made as large as we want (with n > $|\sigma|$). When this difference is sufficiently large (larger than our constant), we get a contradiction.
Update : The proof that such an optimal machine $U$ exists goes as follows: Suppose you have an effective list $\{M_e\}_{e \in \mathbb{N}}$ of all prefix-free machines. Then you define $U(1^e 0 \tau)$ to be $M_e(\tau)$. Note that $U$ is prefix-free. The problem is now to get that list, from an effective list $\{N_e\}_{e \in \mathbb{N}}$ of (not necessarily prefix-free) machines (that we have): To do so, replace each machine $N_e$ with a machine $M_e$ which on input $\tau$ executes $N_e(\tau)$. If we get an answer in $t$ step of computation (we suppose $t > |\tau|$), $M_e$ then verifies that $N_e$ does not also halt in less than $t$ steps, on a string of length smaller than $t$ and comparable with $\tau$. Then $M_e$ adapts its answer to keep prefix-freeness. All that matters is that $M_e$ behaves like $N_e$ if $N_e$ is already-prefix-free, which we get, by design.
Finally, note that in the "real world", machines are prefix-free, for example in any file system, one should know where a file starts and where it ends.
