Final answer:
The best signal word to show cause and effect in the provided sentence is 'therefore,' which connects the cause of Timothy standing in the rain without an umbrella to the effect of getting wet.
Step-by-step explanation:
In the sentence 'Timothy was standing in the rain without an umbrella. ________, he got wet.', the best signal word to show cause and effect is 'therefore'. This is one of the key words used to connect a cause with its effect. An example of using 'therefore' in this sentence would be: 'Timothy was standing in the rain without an umbrella. Therefore, he got wet.' This sentence structure is one where the cause is Timothy standing in the rain without protection, and the effect is him getting wet. 'Therefore' is used to show the direct result of the cause.