Final answer:
The severity of this client's asthma attack can be classified as Moderate based on the assessment findings described.
Step-by-step explanation:
Based on the assessment findings described, the severity of this client's asthma attack can be classified as Moderate. The client is experiencing difficulty breathing while lying supine, talking in phrases without experiencing shortness of breath, and has a moderately elevated respiratory rate. These symptoms suggest that the client's airways are partially blocked, leading to some level of breathing difficulty.