Solve for Missing Addend: 0.3 + ? = 0.8

Let's work through the solution by performing the necessary calculation:

• Step 1: Identify the known values:
  The equation provided is $0.3 + ? = 0.8$.

• Step 2: Use the subtraction operation to solve for the missing number, ?:
  We need to subtract $0.3$ from $0.8$.

Calculation:
$\text{?} = 0.8 - 0.3$

We perform the subtraction as follows:

• Write both numbers with the decimal points aligned:
  $0.8$
  $- 0.3$

• Perform the subtraction:
  $0.8 - 0.3 = 0.5$

Thus, the missing number is $\textbf{0.5}$.

Therefore, the correct answer is $\textbf{0.5}$.