Why should we not use a preposition here?

[Pavan]

English Language Tests, Intermediate level

ESL/EFL Test #203 "Prepositions of Time and Date (1)", question 3

I don't know what time they'll be back ......... this evening, if at all. It all depends on the road conditions today.

(a) for
(b) in
(c) (none)

English Language Tests, Intermediate level

ESL/EFL Test #203 "Prepositions of Time and Date (1)", answer 3

I don't know what time they'll be back this evening, if at all. It all depends on the road conditions today.

Correct answer: (c) (none)

Your answer was: incorrect
I don't know what time they'll be back in this evening, if at all. It all depends on the road conditions today.
_________________________

Why can't we write like this:
I don't know what time they'll be back in this evening, if at all. It all depends on the road conditions today.

why should we not use a preposition here.

Thanks,

Pavan

[Yankee]

Hi Pavan

You can say in the morning, in the afternoon, in the evening.

However, when you use the word this, then you cannot use the word in:
We're going to a party this evening.

Amy