C. Le 了is used to ask about what someone did. The question uses the modal particle le了, which is the sentence-end le了, but the answer must use the verb-suffix le了, the aspect particle, and the object must be quantified in terms of quantity or duration. If the verb is qù去, it usually takes the suffix le了 and a verbal measure word yí tàng一趟(a trip to) before taking the object which indicates a place.
3.1 A: Zuótiān wǎnshang nǐ dōu gàn shénme le?
What did you do last night?
B: Xiě-le yì fēng xìn, kan-le yì huǐr diànshì, hái qu-le yí tàng túshūguǎn.
I wrote a letter, watched a little TV and also went to the library.
The dōu都used in the question is to show expectation of a list of events.
3.2 A: Shàng gè zhōumò nǐ dōu mǎi shénme le?
What did you buy last weekend?
B: Méi mǎi shénme. Jiù mǎi-le yì běn xiǎoshuō.
I didn’t buy much. I only bought a novel.
3.3 A: Wǔfàn nǐ dōu chī shénme le?
What did you eat at lunch?
B: Wǒ chī-le bàn tiáo yú hé yì diǎnr qīngcài.
I ate half a fish and some vegetables.
The verb in this type of what-question can be any action verb that takes an object. The important thing is that this type of question asks for very specific information, so the answer must use verb-suffix le了 and the object must be quantified or modified. The question, however, uses the sentence-end modal particle le了. “The more specific and delimited the action is made by reference to particular context, the greater the need for this verb-suffix –le, to the degree that it is obligatory where precision is given to the object or complement by quantifying it.” (P.C. T’ung and D. E. Pollard: 1982, p. 142)