Daniel can complete a custom landscaping project in the exact same duration it takes Ethan and Fiona working together to finish it. If Daniel and Ethan working together can complete the project in 10 days, and Fiona working alone can complete it in 30 days, how many days will Daniel take to finish the project alone?

The correct option is 15 days.

Given Data:
Let the fraction of work completed in one day by Daniel, Ethan, and Fiona be represented by $D$, $E$, and $F$ respectively.

1. Daniel takes the exact same duration as Ethan and Fiona working together:
$D=E+F$

2. Daniel and Ethan together can finish the project in 10 days:
$D+E=\frac{1}{10}$

3. Fiona working alone can finish the project in 30 days:
$F=\frac{1}{30}$

Step-by-Step Derivation:

From equation (1), express Ethan's daily rate ($E$) in terms of Daniel's rate ($D$) and Fiona's rate ($F$):
$E=D-F$

Substitute Fiona's rate $F=\frac{1}{30}$ into the expression for $E$:
$E=D-\frac{1}{30}$

Substitute this value of $E$ into equation (2):
$D+\left(D-\frac{1}{30}\right)=\frac{1}{10}$

Combine like terms:
$2D-\frac{1}{30}=\frac{1}{10}$

Add $\frac{1}{30}$ to both sides of the equation:
$2D=\frac{1}{10}+\frac{1}{30}$

Take the common denominator of 30 for the fractions on the right side:
$2D=\frac{3}{30}+\frac{1}{30}$
$2D=\frac{4}{30}$
$2D=\frac{2}{15}$

Divide both sides by 2 to solve for Daniel's daily rate ($D$):
$D=\frac{1}{15}$

Since Daniel completes $\frac{1}{15}$ of the total project work in one day, the total number of days Daniel will take to complete the project alone is:
$\text{Time taken by Daniel}=\frac{1}{D}=15\text{days}$

Therefore, Daniel alone will finish the project in 15 days.