This is an Assumption question. The best and most foolproof way to approach an assumption question is to use the Negation Strategy. The fundamental idea is that an assumption is the foundation of an argument, and has to be absolutely true for the argument to be valid. Therefore, if the assumption is NOT TRUE, then there is no way that the argument can be true. In the negation strategy, we negate the options, and see which option invalidates/completely destroys the argument - this option which, when negated, totally destroys the argument, is the correct assumption.

Firstly, let us look at the argument given in this question. The argument states that since toddlers watch more television (more than 2x times) today than two decades ago, they have less developed motor skills. In other words, the argument tries to establish a causal connection between television and slower development of motor skills. The negation of the correct assumption would destroy this causality. Let us go through the negation of the options provided below:

Option A - Television cannot be used to enhance many skills of children aged 1-3 . This option does not talk about motor skills - INCORRECT
Option B - Overall, watching television does not have a negative effect on young children. Same as A - INCORRECT
Option C - Toddlers should not be encouraged to play outdoors more. 'Should' is never correct in an assumption question. Either way, the negation of C doesn't break the linkage between enhanced television viewing and slow development of motor skills - INCORRECT
Option D - Toddlers who watch less television are not engaged in activities which develop their motor skills. Perfect - this says that there is no correlation between TV viewing and development of motor skills. Since the negation of D destroys our conclusion, this must be the correct assumption.
Option E - Only the physical removal of the television set from homes with young children will not enable them to develop their motor skills more quickly. This is irrelevant - we are not concerned with the physical removal of TV sets - INCORRECT.

Hence, since the negation of Option D negates our conclusion, this is our best answer.

Please let me know in case anything doesn't make sense.