Overlap-Add Method
The overlap-add method is a widely used technique in signal processing for implementing linear time-invariant (LTI) systems with finite impulse response (FIR) filters. It is commonly used for efficient convolution of long input sequences with FIR filters.

Length of the Input Data Block
In the overlap-add method, the input data block is divided into smaller segments or frames. The length of each frame is typically chosen to match the length of the filter impulse response.

The correct answer to the question is option 'B' - Lc.

Explanation
To understand why the length of the input data block is Lc, let's break down the overlap-add method.

1. Segmentation: The input data block is divided into smaller segments or frames. Each frame has a length equal to the length of the filter impulse response. Let's denote this length as L.

2. Overlap: To ensure continuity in the output signal, adjacent frames overlap by L-1 samples. This means that L-1 samples from the previous frame are added with L-1 samples from the current frame.

3. Filtering: Each frame is convolved with the filter impulse response to produce an intermediate output sequence. The length of the intermediate output sequence is L + Lc - 1, where Lc is the length of the input data block.

4. Addition: The intermediate output sequences are added together, taking into account the overlapping samples. The resultant output sequence is the sum of all intermediate output sequences.

Therefore, the length of the input data block, Lc, is given by:
Lc = L + (number of frames - 1) * (L - 1)

In the overlap-add method, the input data block is divided into frames of length L, and adjacent frames overlap by L-1 samples. Thus, the correct answer to the question is option 'B' - Lc.