The most abundant of all of the isotopes of an element will be the one who's mass the mass of element is closest to. In this case, the mass of atomic carbon is closest to the mass of carbon-12.
Thus, Carbon-12 is the most abundant isotope.
Answer: 41.5 mL
Explanation:
Molarity of a solution is defined as the number of moles of solute dissolved per liter of the solution.

where,
n = moles of solute
= volume of solution in L
Given : 59.4 g of
in 100 g of solution
moles of 
Volume of solution =
Now put all the given values in the formula of molality, we get

To calculate the volume of acid, we use the equation given by neutralisation reaction:

where,
are the molarity and volume of stock acid which is 
are the molarity and volume of dilute acid which is 
We are given:

Putting values in above equation, we get:

Thus 41.5 mL of the solution would be required to prepare 1550 mL of a .30M solution of the acid