Q.Which of the following isotopes is used for finding the fossil age maximum about 3,50,000 years ?

Palaeontology is the study of fossils (remains/impressions of past organisms). Fossils found in different rock layers show the sequence and gradual change of life forms over time — this is palaeontological evidence for evolution.

Palaeontological evidence refers to fossils — the preserved remains, impressions or moulds of organisms in sedimentary rocks. Studying fossils across successive geological strata reveals which forms existed at which times, transitional forms (e.g. Archaeopteryx), and the progressive change of species, strongly supporting evolution.

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Radiocarbon (¹⁴C) dating, based on the decay of carbon-14 (half-life ~5730 years), is used to determine the age of relatively young fossils/organic remains — the appropriate isotope for a maximum age around a few hundred thousand years.

Different radioisotopes suit different age ranges:

• ¹⁴C (carbon-14): half-life ~5730 years; used for recent fossils and organic material — the correct choice for young fossils (the question's ~3,50,000-year range) as opposed to isotopes used for millions/billions of years.
• ²³⁸U → ²⁰⁶Pb (uranium–lead): half-life in billions of years; used for very ancient rocks/fossils, far older than the range asked. …

Archaeopteryx is the most famous transitional fossil, showing a combination of reptilian and bird-like characters, linking Reptilia to Aves (birds).

Archaeopteryx (found as a fossil from the Jurassic period) is considered the classic "missing link" connecting reptiles and birds because it possesses characters of both groups simultaneously:

• Reptilian features: teeth in the jaws, a long bony tail, and claws on the wings. …