by readability if you mean students ability to read then in English there are levels in which students are tested and marked at each levels .  At least in Canada , they have CLB levels

http://www.cic.gc.ca/english/resources/tools/language/charts.asp

The level that represents a particular row students understanding of written prose text, measured by the average number of correct callbacks by students for the deleted words from the text in accordance with test Claus "

•           Text legibility levels depending on the test results of Claus:

 llmkrueh three levels are:

1-                  Independent level: the level at which the student can read the text and has assimilated without receiving help from the teacher, and determined students to obtain a degree value between (61-100%) in test Claus.

2-                  Educational level: the level at which the student can have text read and assimilated by the supervision of the teacher and his assistant, and determined students to obtain a degree value between (41-60%) In test Claus.

3-                  Frustrating level: the level at which the student has failed to absorb the text and understand it even with the help of the teacher, and determined students to obtain a degree of (40%) in test Claus

I agree with your second answer 

I think readability is a result of clarity of writing expression, freedom from grammatical errors, and a lack of unexplained specialized language. 

The Flesch-Kincaid Grade Level Readability Formula 

Step 1: Calculate the average number of words used per sentence. 

Step 2: Calculate the average number of syllables per word. 

Step 3: Multiply the average number of words by 0.39 and add it to the average number of syllables per word multiplied by 11.8. 

Step 4: Subtract 15.59 from the result. 

The specific mathematical formula is: 

FKRA = (0.39 x ASL) + (11.8 x ASW) - 15.59 

Where, 

FKRA = Flesch-Kincaid Reading Age 

ASL = Average Sentence Length (i.e., the number of words divided by the number of sentences) 

ASW = Average number of Syllable per Word (i.e., the number of syllables divided by the number of words) 

Analyzing the results is a simple exercise. For instance, a score of 5.0 indicates a grade-school level; i.e., a score of 9.3 means that a ninth grader would be able to read the document. This score makes it easier for teachers, parents, librarians, and others to judge the readability level of various books and texts for the students. 

Theoretically, the lowest grade level score could be -3.4, but since there are no real passages that have every sentence consisting of a one-syllable word, it is a highly improbable result in practice.

http://www.readabilityformulas.com/flesch-grade-level-readability-formula.php

dear

this is my opinion about Readability

Readability is the ease with which a reader can understand a written text. In natural language, the readability of text depends on its content (the complexity of its vocabulary and syntax) and its presentation (such as typographic aspects like font size, line height, and line length).

https://en.wikipedia.org/wiki/Readability

regards