Step 1 (Understand the sentence meaning).
The sentence describes the weather in a positive way, leading to the decision to have a picnic. Therefore, a positive adjective is needed.
Step 2 (Check each option).
(a) pleasant — fits well, describing nice weather.
(b) angrily — incorrect, negative emotion and not suitable for weather description.
(c) quickly — adverb, incorrect for describing weather.
(d) slowly — adverb, not a quality of weather here.
Step 3 (Conclusion).
The correct sentence is: "The weather was so pleasant that we decided to have a picnic in the park."
\[
{\text{pleasant (Option (a)}}
\]