The correct answer is option 1, I only. The main factor responsible for the high concentration of jute mills in the Hugli basin is the nearness to coalfields. Jute mills require a large amount of coal to power their machinery, and the proximity to coalfields allows for easy accessibility and reduced transportation costs. This makes it economically feasible for jute mills to operate in the Hugli basin.

On the other hand, the convenient dry climate for spinning and weaving mentioned in option II is not a significant factor in the concentration of jute mills in the Hugli basin. While dry climate can be beneficial for certain textile industries, it is not a crucial requirement for jute mills. Other factors such as the availability of raw jute and transportation facilities are more important considerations for the location of jute mills.

In summary, the high concentration of jute mills in the Hugli basin is primarily due to the nearness to coalfields, making option 1 the correct answer. The convenient dry climate for spinning and weaving mentioned in option II is not a significant factor in this context.