**Red algae and brown algae:**
- Red algae and brown algae belong to different algal groups and have different pigment compositions.
- Red algae contain a pigment called phycoerythrin, which gives them their characteristic red color. Phycoerythrin absorbs blue and green light and reflects red light.
- Brown algae, on the other hand, contain a pigment called fucoxanthin, which gives them their characteristic brown color. Fucoxanthin absorbs blue and green light and reflects brown light.

**Green algae and blue-green algae (cyanobacteria):**
- Green algae and blue-green algae (cyanobacteria) also belong to different algal groups and have different pigment compositions.
- Green algae contain chlorophyll a and b, which are the primary pigments responsible for photosynthesis. These pigments absorb light in the blue and red regions of the spectrum.
- Blue-green algae (cyanobacteria) do not contain chlorophyll b, but instead, they contain chlorophyll a and phycocyanin, which gives them their characteristic blue-green color. Phycocyanin absorbs blue light and reflects green light.

**Kelps and diatoms:**
- Kelps and diatoms belong to different algal groups and have different pigment compositions.
- Kelps, which are a type of brown algae, contain fucoxanthin as their primary pigment, which gives them their characteristic brown color.
- Diatoms, on the other hand, contain chlorophyll a and c, as well as a brown pigment called fucoxanthin. Fucoxanthin gives diatoms a brownish color, similar to kelps, but their overall pigment composition is different.

**Diatoms and euglenoids:**
- Diatoms and euglenoids belong to different algal groups and have different pigment compositions.
- Diatoms contain chlorophyll a and c, as well as a brown pigment called fucoxanthin, which gives them their characteristic golden-brown color.
- Euglenoids, on the other hand, contain chlorophyll a and b, as well as a red pigment called astaxanthin, which gives them their characteristic red color. Astaxanthin absorbs blue and green light and reflects red light.

Therefore, the correct answer is option 'C' - Kelps and diatoms have similarity in pigment composition. Both kelps and diatoms contain the brown pigment fucoxanthin, which gives them their characteristic brown color.

Kelps belong to brown algae which carry almost the same pigments as in diatoms. one of them is fucoxanthin.