Answer: (a) Plants do not exhibit visual symptoms of deficiency due to the short supply of particular nutrients but will result in yield losses.
Explanation: Hidden hunger is a condition where plants or humans suffer from a lack of essential micronutrients, but do not exhibit any visible symptoms of deficiency. Option (a) correctly describes hidden hunger, while the other options do not. Option (b) describes gravitropism, option (c) describes herbicides, and option (d) describes photomorphogenesis.