Muchachas colombianas son bonitas.
a.
el
c.
los
b.
la
d.
las

The right answer is d. Las

Definite articles are used to talk about specific nouns. In Spanish, there are four definite articles: el, la, los, and las and they often all translate to the same word in English: the. Nouns are often accompanied by articles. So the use of them points out the gender (either masculine or feminine) and number (either singular or plural) of a noun. In the Table below is shown the four forms that Spanish definite articles take. There are some exceptions to this rule. For example, if a feminine singular noun starts with a stressed a or ha sound then we use the masculine definite article rather than the feminine definite article.

Therefore:

Las muchachas colombianas son bonitas (Colombian girls are pretty)

can you explain this question better?

context? topic?