Step 1: Define variables and set up the equation
Let b be the number of boys and g be the number of girls in the class.
Total marks of boys = 52b.
Total marks of girls = 42g.
Average marks of the class = 50.
Equation:
\[
\frac{52b + 42g}{b + g} = 50
\]

Step 2: Simplify the equation
Multiply both sides by (b + g):
\[
52b + 42g = 50b + 50g
\]

Step 3: Isolate variables
Subtract 50b from both sides:
\[
2b + 42g = 50g
\]
Subtract 42g from both sides:
\[
2b = 8g
\]

Step 4: Find the ratio of boys to girls
Divide both sides by 2:
\[
b = 4g
\]
Ratio of boys to girls is 4:1.

Step 5: Calculate the percentage of boys
Total students = b + g = 4g + g = 5g.
Percentage of boys = \(\frac{b}{b+g} \times 100 = \frac{4g}{5g} \times 100 = 80\%\).

Answer:
The percentage of boys in the class is (A) 80%.