Correct Answer: Upturn

Solution : Given:
Nature, Ensure, Tense, Spent, Spurn,?

In the given series, note the position of the letter N in each given word.
N ature (1st place), E n sure (2nd place), Te n se (3rd place), Spe n t (4th place), Spur n (5th place).
Therefore, the next word will be the one that contains the letter N in the 6th place.

Let's check the options –
First option: Pushup; Here, the letter N is not present in the word.
Second option: Thrash; Here, the letter N is not present in the word.
Third option: Upturn; Here, the letter N is in 6th place.
Fourth option: Asset; Here, the letter N is not present in the word.

So, the third option follows the same pattern as the given series. Hence, the third option is correct.