Which of the following sentences is capitalized correctly according to standard English conventions?

Introduction / Context:
This question examines your ability to apply capitalization rules correctly. The sentence talks about observing the constellation Pisces in the night sky. You must choose the version that uses capital letters appropriately for sentence beginnings, proper nouns, and common nouns, while avoiding unnecessary capitals.

Given Data / Assumptions:
- The sentence structure is "On a cloudless night, I studied Pisces, a constellation in the northern sky." - Proper nouns like Pisces and the pronoun I should be capitalized. - The first letter of a sentence should be capitalized. - General directional adjectives such as northern, when used in phrases like northern sky, are usually written in lowercase.

Concept / Approach:
Standard capitalization rules state that the first word of a sentence must begin with a capital letter. The personal pronoun I is always capitalized. Proper names of constellations such as Pisces are capitalized, since they are specific names. However, in the phrase northern sky, northern acts as a common adjective, not a proper noun referring to a region like the North, so it is normally written with a lowercase initial. Applying these rules allows us to check each option systematically.

Step-by-Step Solution:
Step 1: Inspect option a: "on a cloudless night, I studied pisces, a constellation in the north sky." The first word on is not capitalized, which is incorrect. The constellation name pisces is in lowercase, which is also incorrect. The phrase north sky is not the usual expression, as we prefer northern sky. Step 2: Look at option b: "On a cloudless night, I studied Pisces, a constellation in the northern sky." Here, On begins with a capital letter, I is capitalized, Pisces is capitalized as a proper noun, and northern is correctly left in lowercase as a general descriptive term. Step 3: Consider option c: "on a Cloudless Night, i studied Pisces, a constellation in the North Sky." This sentence capitalizes Cloudless, Night and North and leaves i in lowercase, all of which violate standard rules. Step 4: Examine option d: "On a cloudless night, I studied pisces, a constellation in the north sky." The sentence correctly starts with On and correctly capitalizes I, but it keeps pisces in lowercase, which is wrong for a constellation name, and uses north sky instead of northern sky. Step 5: From this analysis, option b is the only one that follows capitalization rules correctly and uses the standard phrase northern sky.

Verification / Alternative check:
Compare the treatment of other constellation names in textbooks, such as Orion, Scorpius or Leo. These are always capitalized, just like country names. Also check sentences like "On a warm evening, I walked along the river." The first letter On is capitalized, while common adjectives like warm remain lowercase. Similarly, directional adjectives in phrases like southern ocean current or western horizon are lowercased. This pattern matches what we see in option b and confirms its correctness.

Why Other Options Are Wrong:
Option a: Fails to capitalize the first word and the proper noun Pisces. Option c: Over-capitalizes common words such as cloudless and night, leaves i in lowercase, and capitalizes North unnecessarily. Option d: Correctly begins the sentence and capitalizes I but fails to capitalize Pisces and uses the less natural phrase north sky.

Common Pitfalls:
Students often confuse when to capitalize directional words. North, South, East and West are capitalized only when used as proper names for regions, such as the North or the West. When they function as mere directions or adjectives, they remain lowercase. Similarly, they sometimes forget that specific astronomical objects like Mars, Jupiter and Pisces are treated as proper nouns. Reviewing these rules helps avoid losing marks on relatively easy capitalization questions.

Final Answer:
The sentence capitalized correctly is "On a cloudless night, I studied Pisces, a constellation in the northern sky."