If you have $0$ clients on Monday, and $5$ clients on Tuesday, how many times have the number of clients you had grown from Monday to Tuesday?
$A$ - Infinite times
$B$ - Unknown
$C$ - Undefined
If you have $0$ clients on Monday, and $5$ clients on Tuesday, how many times have the number of clients you had grown from Monday to Tuesday?
$A$ - Infinite times
$B$ - Unknown
$C$ - Undefined
To ask "how many times have the number of clients you had grown from Monday to Tuesday" is the same to ask what is the value of: $\dfrac{\mathrm{Clients}(\mathrm{Tuesday})}{\mathrm{Clients}(\mathrm{Monday})}=\frac50$
In the context of the real numbers, or natural numbers if you prefer, this is undefined. We cannot divide by the actual number zero.
Note that it is common to say "infinity" because $\lim\limits_{n\to0}\frac5n=\infty$. However this simply tells us that the ratio is larger than any other number, it is not an actual number or ratio per se.
Go with "c - Undefined".
Translating the word problem to a precise statement I find it to mean the following:
Let $x = 0$ and $y=5$. If $y=kx$, then what is $k$?
Solving this equation would involve division by zero, so the answer is undefined, i.e. there is no such $k$.