The reagent 'X' must be capable of performing both transformations:

1. Reaction 1 (Reduction of Carboxylic Acid): Carboxylic acids are reduced to primary alcohols by strong reducing agents like Lithium Aluminium Hydride ($LiAlH_4$) or Diborane ($B_2H_6$). notably, $B_2H_6$ is a versatile reagent that reduces carboxylic acids even more readily than esters or ketones. Sodium Borohydride ($NaBH_4$) does not reduce carboxylic acids.
2. Reaction 2 (Hydroboration-Oxidation of Alkene): The conversion of an alkene ($R-CH=CH_2$) to a primary alcohol ($R-CH_2-CH_2-OH$) anti-Markovnikov addition of water requires Diborane ($B_2H_6$) followed by oxidation with alkaline hydrogen peroxide ($H_2O_2/OH^-$). $LiAlH_4$ does not effect this transformation.

Since 'X' must be common to both, $B_2H_6$ is the correct answer.