The postposition का is a very common postposition. It is unique because it also behaves like an adjective: it inflects and modifies a noun (sometimes called the “head noun”).
However, it can also be used independent of any “head noun”. This is similar to the independent use of the word “of” in English. It is a colloquial alternative to के बारे में (“about”), etc.
कौन सा फ़ोन खरीदने का सोच रही हो – “Which phone are you thinking of buying?” (Spoken to a female)