Which one among the following is the major cause of blurring and unsharp images of objects observed through very large telescope at the extreme limit of magnification ?

The blurring and unsharpness of images in large ground-based telescopes at high magnification are primarily caused by air turbulence in the Earth's atmosphere, a phenomenon known as 'astronomical seeing' [2]. As light from celestial objects enters the atmosphere, it passes through different temperature layers and wind speeds that act as turbulent cells of air [1]. These pockets of air have varying temperatures, which directly affect their density and refractive index [2]. This causes the light waves to distort and the image to 'dance' or blur, creating a 'seeing disc' rather than a sharp diffraction-limited point [1]. While varying density is the physical mechanism, the dynamic 'turbulence' is the specific cause of the blurring effect [1]. Large telescopes are particularly affected because their theoretical resolution is much higher than the limit imposed by the atmosphere, which is why they are often placed on high mountains or in space to bypass these turbulent layers.

Sources

1. [1] https://en.wikipedia.org/wiki/Astronomical_seeing
2. [2] https://vikdhillon.staff.shef.ac.uk/teaching/phy217/telescopes/phy217_tel_atmos.html