If you see any type of cloud, you can expect fair weather. cirrus altonimbus nimbus stratus

Option A, Cirrus

Explanation:

Cirrus are thin clouds found at greater heights (some 25000 feet above the ground). They are made up of super cooled water and generally represents fair weather.  

While Stratus cloud leads to rain when it gets warm and snow when its gets cold, so the weather is not fair when there are  Stratus cloud in the sky

Nimbus cloud also causes rain, snow or other form of extensive precipitation and hence they are also not responsible for producing fair weather condition

Altonimbus is a mid level cloud which also causes precipitation in some or the other form.  

Hence option A, Cirrus is the correct answer among all given option

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.