In the initial period of learning, describes when an organism learns to connect a neutral stimulus and an unconditioned stimulus.

The given blank can be filled with acquisition.

Explanation:

An unconditioned stimulus provokes the unlearned response and an unconditioned response is the naturally taking place reaction. A neutral stimulus is illustrated as the stimulus that prior to conditioning do not generate any kind of response.  

A type of learning wherein a subject begins to react towards neutral stimulus as it would perform with another stimulus by acquiring the path of associating the two stimuli is termed as classical conditioning.  

An acquisition can be illustrated as the time of learning in classical conditioning where an individual begins to associate a neutral stimulus with an unconditioned stimulus that further permits the neutral stimulus to begin possessing the conditioned response.  

oxygen = 21% of the atmosphere

nitrogen = 78%

carbon dioxide = 0.1%

argon =0.934%