The focal length of a convex lens is 25 cm. Find its power with a sign:
A) +4 D
B) -4 D
C) +2.5 D
D) -2.5 D

To solve the problem, we use the formula for the power of a lens:

**Power (P) = 100 / f**,
where **f** is the focal length of the lens in centimeters.

### Given:
The focal length of the convex lens, **f = 25 cm**.

### Step-by-Step Solution:
1. Convert the focal length into meters:
\( f = 25 \, \text{cm} = 25 / 100 = 0.25 \, \text{m} \).

2. Calculate the power using the formula:
\( P = 100 / f \)
\( P = 100 / 25 = +4 \, \text{D} \).

3. The sign of the power is positive because the lens is convex, which converges light rays.

### Final Answer:
**Option A) +4 D**