Take an umbrella ____ it rains ?

The phrase "Take an umbrella ____ it rains" requires a connector that indicates a precautionary action in anticipation of a possible future event. Among the given options, "in case" is the most suitable choice.

"In case" is a phrase that implies preparation for a potential occurrence. In this context, when someone says "Take an umbrella in case it rains," they are suggesting that the use of the umbrella is a proactive measure to prepare for the possibility of rain, which may or may not happen.

Let's briefly consider the other options:

A) "so that" implies a purpose or goal; it would work better in a sentence like "I took an umbrella so that I wouldn't get wet," suggesting that the speaker took the umbrella to prevent being wet, rather than as a precaution.

C) "so" indicates a reason or consequence, and it does not fit the conditional precaution needed in this scenario.

D) "_" implies no addition; therefore, it lacks meaning in this context.

Thus, "in case" effectively conveys the notion of being prepared for the likelihood of rain, making it the correct choice.