Adecimal outline is an outline that uses notations such as 1.1, 1.2, and 1.3 i, ii, and iii a, b, and c a, b, and c

A decimal outline is an outline that uses notations such as 1.1, 1.2, and 1.3. The second outline is Roman numerals while the third is Letter notation.

The correct answer is: 1.1, 1.2, and 1.3

Explanation:
A decimal outline is an outline that uses notations such as 1.1, 1.2, and 1.3. In decimal outline, the notations have to be numbers, not letters or roman numerals.

Examples of Decimal Outline:
1. 1.1, 1.2, 1.3...
2. 2.1, 2.2, 2.2...

The other options, however, are either roman numerals or the letters.
Like:
Roman numerals: I, II, and III 
Letters: A, B, and C; a, b, and c

Hence the correct answer is 1.1, 1.2, and 1.3.