Assertion: $ {\mathop A\limits^ \circ } $ (Angstrom) and AU are different units of length.
Reason: $ {\mathop A\limits^ \circ } $ (Angstrom) is a small unit of length while AU is a big unit of length.
(A) Both assertion and reason are correct and the reason is the correct explanation for the assertion.
(B) Both assertion and reason are correct but the reason is not the correct explanation for assertion
(C) Assertion is correct but the reason is incorrect
(D) Both assertion and reason are incorrect

Hint: In the assertion, we have two units called Angstrom and AU. Angstrom is mostly seen while calculating quantities in the atomic levels like the atomic radius. The other unit is AU that is the astronomical unit used in astrophysics for measuring astronomical values.

Complete step by step answer
Angstrom is used to measure the length at an atomic level. The value of one angstrom is given as, $ 1{A^ \circ } = {10^{ - 10}}m $ . It is used to denote very small values of length. It is mostly used in atomic and nuclear physics.
At the same time, the astronomical unit is used in astrophysics to measure the lengths on an astronomical scale. It is a very large quantity. The value of one astronomical unit is given by
 $ 1AU = 149597870700m $ . From this, we know that an Astronomical unit is also a unit of length.
So we can say that both the angstrom and astronomical unit are units to measure length. But one is on the microscale and the other is on the macro scale. Therefore we can say that both the assertion and the reason are correct and the reason is the correct explanation for the assertion.
The answer is: Option (A): Both assertion and reason are correct and the reason is the correct explanation for the assertion.

Note
There are many different units to measure lengths in different scales. The length varies from very small micro values to millions of kilometers. Therefore we use different units depending on the value of the length. For very large values like astronomical distances we use units like AU, parsec, light year, etc., and for very small values like nuclear radii, we use units like angstrom, nanometer, etc.