Why is The sun is the ultimate source of energy
for all living organisms.'

"Sun is the ultimate energy source for all living organisms."

✴This is because plants need sunlight to make food through photosynthesis. This food is consumed by the animals and human-beings. Carnivores indirectly depend on plants for their food. So, if Sun is not there there will be no life on earth.

✴Secondly, sunlight is a good source of Vitamin-D.

Vitamin D is essential for strong bones, because it helps the body use calcium from the diet. Traditionally, vitamin D deficiency has been associated with rickets, a disease in which the bone tissue doesn't properly mineralize, leading to soft bones and skeletal deformities. So if sunlight is not there it can result in loss of Vitamin-D which can cause deficiency diseases.

✴If sunlight is not there there will be no day on Earth. Light will not reach our Earth. Every time there will be only darkness.

Answer From Gauth Math

that is true   : ) hope this you