Comparison Vigènere vs. Monoalphabetic cipher
a) The question seems to be about a comparison of the size of the key spaces. The hint already shows the key spaces for Vigenère with a 10-letter key ($26^{10}$) and for simple substitution ($26!$). Simple substitution has the much bigger key space.

b) Using frequency analysis simple substitution is much easier to solve. Basic frequency analysis does not help for Vigenère (at least not without additional methods). With that in mind Vigenère could be considered more secure than simple substitution. In fact: both are not really secure, don't rely on them.

c) Vigenère with a key that is exactly as long as the plain text could be considered absolutely secure. But then again, it would be highly unpractical. :-)