Which of the following is the most precise device for measuring length?

Which of the following pairs is incorrect?

$(a)$ How many astronomical units $(AU)$ make $1$ parsec?
$(b)$ Consider a sun-like star at a distance of $2$ parsecs. When it is seen through a telescope with $100$ magnification,what should be the angular size of the star? The Sun appears to be $(\frac{1}{2})^{\circ}$ from the Earth. Due to atmospheric fluctuations,the eye cannot resolve objects smaller than $1$ arc minute.
$(c)$ Mars has approximately half of the Earth's diameter. When it is closest to the Earth,it is at about $\frac{1}{2} AU$ from the Earth. Calculate what size it will appear when seen through the same telescope.

What is mass? Write the effect of external factors on mass.

Explain the method for the measurement of the diameter of a planet or star.

The angular diameter of the Sun is measured to be $1920''$. If the distance of the Sun from the Earth is $1.496 \times 10^{11} \ m$,calculate the diameter of the Sun. (Given: $1'' = 4.85 \times 10^{-6} \ rad$)