C. Yesterday Alla got a cold because ... 1) she didn't have an umbrella. 2) she couldn't use her umbrella. 3) it wasn't warm. 4) it wasn't cool.

If Alla got a cold, the most logical reason from the options provided is that "it wasn't warm". Not having an umbrella or not being able to use one, are potential reasons to get wet, but not directly to get a cold. It being not cool is also less probable. Getting cold can be related to whether it wasn't warm. __Ответ:__ 3) it wasn't warm. Вчера Алла простудилась, потому что не было тепло. Это самый логичный ответ из предложенных.